I think you can really tell when an emcee has written to a a particular beat, as opposed to when they've written a verse and then attempted to "fit" it to a beat. The former seems to work out much better.
That's pretty much how I started rapping....riding around in the car, and just absolutely getting lost in the beat. High school was all about riding around, getting lifted, and zoning. It was in that exact scenario that I heard Afu-Ra's debut album (pretty tight, check it out) for the first time in my buddy's truck. DJ Premier produced a bunch of tracks on it, and one in particular featuring M.O.P., just absolutely blew me away. In the same way that people tap their feet, or bob their head, I just started "scatting" over the beat...it wasn't even all that deliberate. I was just feeling the beat so hard, I felt like I needed to contribute, or be a part of it...as corny as that sounds.
From that point on, I would constantly just ride around and spit shitty freestyles for my own entertainment.
Anyone interested in fooling around and rapping...not that I'm some great rapper who has a wealth of information or anything, I recommend memorizing the lyrics of a bunch of dfferent songs, and "performing" them. In the car is the perfect place (warning: getting caught rapping overly sincerely at a red light by people can be horribly embarrassing. Watch out for that.). After you get a feel for the cadence, start "scatting"— meaning, "dun dun da ba da dun dun dah". From there, just play around with phrases and basic word combinations. Find your own voice, and let it rock sooooooooonnnnnn.
It's ALL about confidence. 99% of spitting is simply believing that your shit is hot. There is nothing worse than a timid emcee. If you can convince yourself that it's dope, you can probably convince other people too.