you sound like you probably have a lot of questions about DSLR filming, so here is a guide I wrote a while ago. It was directed at a specific person so you'll have to ignore some of it. Hopefully it will help clear everything up for you!
There might be a couple mistakes in it at this point and of course much of it is subjective and varies from filmer to filmer, but in general it should answer a lot of your questions.
Every lens has an aperture, or iris (although you don't hear iris so much these days). The aperture is a metal circle comprised of a series of blades inside the lens that opens and closes. The wider it opens, the more light comes through the lens to the camera. The more it closes, the less light comes through.
Aperture is measured in f-stops. Now your current lens is the 18-55mm f/3.5-5.6. You probably know that 18-55mm is the focal length. Your lens will have a focal length of 18mm when it is zoomed out, and a focal length of 55mm when it is all the way zoomed in. A lens with a focal length of 14mm will be wider than your lens, and a lens with a focal length of 85mm will be more zoomed in, or "tighter," than your lens. But you are probably wondering what f/3.5-5.6 means. Well, f/3.5 means an f-stop value of 3.5. When you see that number next to a focal length, that is the widest the aperture can open. For example, I have a 17-55mm f/2.8. So that means that the aperture can open all the way to f/2.8. The lower the f-stop value, the wider the lens can open. So f/1.2 is going to open a lot more than f/4. Your current lens has two f-stop values next to the focal length (f/3.5-5.6). What this means is that when your lens is zoomed out (at 18mm), the aperture can open as wide as f/3.5. But when you're zoomed in all the way to 55mm, the aperture can only open to f/5.6, so it will be more closed.
Most lenses can close up to about f/22. That is very closed, and I rarely ever close the aperture that much. The reason why they only advertise how much the aperture can open and not how much it can close is because how much it can open is more important. This tells you how a lens will perform in low light, because a wider aperture = more light coming through the lens. So a 50mm f/1.2 will be better in low light than the 50mm f/1.8 since the aperture opens wider, letting more light through the lens. When I said "having your lens open," I was referring to opening your aperture all the way.
Additionally, the aperture affects depth of field. Depth of field refers to how much of your image is in focus. A shallow depth of field means much of your image is blurry, with only a small area in focus (such as a picture of a flower petal with a very blurred background). This can be used to really make your subject pop from the background. A deep depth of field means much of the image is in focus (in a car chase scene, you would want a lot of the image to be in focus, so you would use a deep depth of field). The wider your aperture opens, the more shallow your depth of field is. Why that happens, I honestly have no clue, but it does. So your lens opened up to f/3.5 will create an image with a much shallower depth of field than if your lens is closed down to f/11. For lifestyle footage, I like to have my lens more open (somewhere under f/6). For sports, I prefer to be anywhere from f/6-f/11.
It is extremely fun to open your aperture all the way and get shots with shallow depth of field. But as I said, this is a common thing for beginners to do in every shot and it is frowned upon. You want to learn how to use both shallow and deep depth of field to make your shot better; for some shots shallow is good, for others deeper is good. Also, your lens will not produce its sharpest image when the aperture is opened all the way. Most lenses are at their 'sweet spot' for sharpness at around f/6; that f-stop will give you the sharpest image. Once again, I'm not exactly sure why this is, but that's just what happens.
A couple other notes about aperture: You will often hear people say "What's your exposure?" When they say that, they are referring to your f-stop value. For some reason, the word exposure has become synonymous with how wide the lens is open. Also, the more your lens opens, the more the blades of the aperture form a perfect circle. As you close the aperture, the blades come together and form more of a hexagonal shape, which can be seen in your images. Here is a good example of that: http://photo.net/equipment/canon/ef50/
As you can see, the shape of the blurred lights produced by the lens with the wider aperture (f/1.4) is more circular, and the shape of the lights produced by the lens with the more closed aperture (f/1.8) is more like a hexagon. One more thing: the way the lens makes the image look when it is out of focus is called bokeh. Some lenses have very smooth bokeh (meaning the out of focus parts blend together and look very nice), and others have rough bokeh (for whatever reason, the out of focus parts are just ugly). It just depends on the glass.
2. Shutter speed
I actually just copied and pasted this one from a message I wrote to somebody else:
Shutter speed for video can be a confusing concept at first, but its actually very simple once you wrap your head around it. just think about it, all a video is is a bunch of pictures. each pictures needs a shutter speed and aperture. the thing is, with a DSLR the shutter speed has to be equal to or higher than the frame rate (otherwise it wouldn't be able to take enough frames to keep up with the frame rate). so the lowest your shutter speed can be is 1/60 if you're shooting 720p60p. and if you go higher than that, there will be times between frames when the shutter 'closes' and stays 'closed' until it's time to take another frame (not that this is a bad thing at all, just explaining how it works). of course when you're shooting video it's not like an actual shutter is opening and closing.
A lot of people follow the 180 degree rule, which means your shutter is double your frame rate. a variant on this rule is that the shutter speed can be any multiple of the frame rate. so if you're shooting 720p60p and following the 180 degree rule, you would want your shutter speed to be 120. this is a supposed to give a nice feel to the footage. slower shutters (under 250) will give your images motion blur and produce a film-like image, which is very aesthetically pleasing. higher shutters (over 250) will give you crisper slow motion but produces more digital looking footage. so it's really up to you. i use higher shutter speeds for action and lower ones when filming lifestyle. Avoid going over 750, and if you can, avoid going over 500. Very high shutter speeds will give your image a bit of a choppy feel.
ISO is how sensitive your camera's sensor is to light. It developed from film (I think in film it's called ASA, I can't really remember), but it referred to how sensitive the film was to light. Film with a lower ASA is less sensitive to light. It is used more for portrait photography and photography in well-lit situations. Because it is less sensitive to light, it requires a slower shutter speed and wider aperture in order to get enough light. Film with a higher ASA is used for night photography and sports photography. It is more sensitive (which makes it good for night), so the aperture can be closed more and the shutter speed can be higher, which makes it also good for sports. However, film with a higher ASA has much more grain in the image. Film with a lower ASA has less grain and therefore creates a cleaner, nicer looking image.
The same goes for ISO. The higher your ISO, the more sensitive your camera sensor is to the light, creating more noise (noise is the digital form of grain) in your image, and creating an uglier image. Therefore, you want your ISO to be as low as it can be while still letting in enough light, because this will give you a nicer looking image. For daytime, you should always be able to get away with an ISO of 100-200. The only time you would ever need to go higher in daylight is if you have your aperture closed a lot and your shutter speed way up. If you're indoors, you will be very lucky to get away with an ISO of 400. Most likely you will have to go up to 800. Avoid going above 800 if possible.
4. Camera Settings
You want your sharpness all the way down because the sharpness feature is kind of a gimmick that will only make your image uglier, that's the best way I can explain it. You want your contrast turned down because as far as video goes, the DSLR sensors are very fragile. By that I mean they have a hard time handling color in video. By upping the contrast, the sensor will produce weird and unattractive colors. For example, if you film in very bright sunlight, the shadows will be way too dark and the sky could be way too bright. Plus skin tones can turn out very orange. If you turn the contrast down, more image detail will be captured (more detail in the dark and light parts of the image). While your image won't look as cool coming out of the camera, it will allow you to do more with the colors once the shots are on your computer. Saturation isn't as big of an issue to me, which is why I only have it at -1 instead of all the way down at -4 (which is what my contrast is at). And obviously you want to keep your color tone neutral. As far as picture style goes, that's up to you, I use the Neutral picture style.
If you don't know how to access the menu that allows you to control all these things, I will tell you how to do it on a T2i. I assume it's the same on the T3i, but I'm not positive. Hit the menu button, then go to the red icon on top that looks like a photo camera with two vertical dots next to it. In that menu screen, there should be a setting called 'Picture Style.' Scroll down to that and click OK. After that, scroll down until you see a setting called 'User Def. 1.' These allow you to create a custom picture style. To create a custom one, make sure User Def. 1 is highlighted and click the Disp button. You should play around with create different styles to find what you like, but this is mine:
Picture Style: Neutral
Color tone: 0
To set it as your picture style, just press Menu 3 times until you're back to the shooting screen.
-ISO, Shutter Speed, Aperture
First, make sure that you go to your camera menu and turn Movie Exposure to MANUAL.
In a perfect world, it would be possible to keep ISO and shutter speed constant and adjust aperture to get the depth of field you want, all while maintaining the exact same exposure (when i say exposure here I mean level of light, not aperture). Obviously, this is not possible. Many filmers keep their shutter speed constant throughout a shoot so the image always 'feels' the same in terms of the way movement looks. This is a good way to do it. Assuming your lighting conditions don't change, you should be able to keep your ISO constant, or maybe only change it by one ISO level (i.e. changing it from 100 to 200, which won't be very noticeable). So considering that your shutter speed and ISO are constant, you control exposure with your aperture. Unfortunately, this makes it a little difficult to always get the depth of field you desire (for example, you want a shallow depth of field but because it's bright out you have to close your aperture a little, making your depth of field deeper). But that is basically how most filmers do it: try to keep ISO and shutter speed constant, control exposure with aperture.
-Resolution & Frame Rate
When filming lifestyle/short film/documentary work, I like to shoot at 1080p24p. 24p is the cinema standard frame rate, so it gives your footage a film-like look. It may not seem super smooth to you, but it is the most professional frame rate. And the reason for shooting at 1080p is pretty obvious. For action sports, I shoot at 720p60p. However, I don't edit in 60p. When I put my footage on my computer, I use a program called Cinema Tools (which comes with Final Cut Studio) to conform my footage to 24p. If you haven't heard of conforming, it can be a little confusing at first. But think of it this way: Say you record a 1 second clip at 60p (60fps). When you put it on your computer, it will play back at 60p. So it will take 1 second to play back all 60 frames. But when you conform it to 24p, you are changing the clip's data so that it plays back at 24p (24fps). So in the first second it will play the first 24 frames of the clip, in the second second it will play the next 24 frames of the clip, and in the next .5 seconds it will play the remaining 12 frames. Not only does this mean that your clips plays back at the professional looking frame rate of 24p, but it will also give you crystal clear slow motion at 40% of real time. It also means that your lifestyle and sports footage will all be at 24p, which will make editing easier. You can also conform footage that has been shot at 30p to 24p. In the first second it will play the first 24 frames, then in the next .25 seconds it will play the remaining 6 frames. This will give you great slow motion at 80% of real time. You can conform in After Effects as well I believe, but I don't know how.
Some more professional filmers utilize all 3 frame rates: 60p, 30p, and 24p. They film things that they know for sure won't be in slow motion at 24p. They film things that they want in only slight slow motion at 30p, so that when they conform to 24p those clips will play at 80% of real time. They film things that they want in heavy slow motion at 60p, so that when they conform to 24p those clips will play at 40% of real time. Keep in mind that even though your clips play back at 40%, you can always speed them up in your editing program if you don't want them slow. This is why I film all sports at 720p60p and conform to 24p; I just speed them up in Final Cut if I don't want them slow. If you speed up a 60p clip that has been conformed to 24p up to 256.1%, it will play back at real time.
You are probably familiar with white balance after using your VX1000. But just in case you aren't, white balance refers to making sure the whites in your image are true white. Adjusting your white balance correctly will ensure that all of your colors are accurate. White balance is measured in kelvins. The lower the 'temperature,' the cooler the colors will look. The higher the temperature, the warmer the colors will look. I actually don't remember how to adjust white balance on a stock T2i (I use a different, non-stock firmware on my T2i, so it operates a little differently), so you will have to google how to adjust white balance on the T3i.
Here's a quick look at how the buttons work on the T2i; hopefully it's the same on the T3i, but once again I don't know. When you're in video mode, shutter speed is easily changed by turning the wheel by the shutter button. Aperture is changed by holding down the AV+/- button and turning the wheel by the shutter button. ISO is changed by pressing the ISO button, then turning the wheel by the shutter button. Press the ISO button again to set it.
There are 3 filters you should have: UV, Circular Polarizer, and ND. UV filter is just a clear filter that is really only used to protect your lens. A circular polarizer is a filter that will make the sky a richer blue, and will also cut reflections out of water and windows. The amount that you turn the filter affects the amount of polarization. The ND filter cuts down the amount of light that comes into the lens. These are best used in the situation I was talking about earlier: it's bright outside, you want to keep your ISO and shutter speed constant, but you also want shallow depth of field. When you put the ND filter on, it will cut down the amount of light, allowing you to open your lens wider. I have a .6 ND filter that I use for skiing because the snow is so bright.