The most easy grab to start with is either safety, blunt, or stalefish.
While safety is one of the easiest to do, I wouldnt do them much. they look good all tweaked out and stuff and with cork 3s but at your level its hard to do much with a safety. I would learn them and then do them in a 180 and once you have that down I would start doing other tricks a bit more.
Blunt and Stalefish are also easy even though many disagree, but heres why they are truuuuly beginner grabs. they are the most effortless to do. Watch practically any edit and you will see about 500 shots of someone doing a cork 5 blunt. What blunt and stalefish have over other grabs is that you just have to point your toes down and reach back a bit. Other grabs you have to lean forward, stretch your arms, or bend over. all there is to these 2 is pointing your toes down and most of the work is done.
The reason I include stale fish with blunt is that stalefish can be easier for those who are challenged with crossing their skis with control and figuring out where each ski is. for stalefish, you just dont cross your skis and tweak your skis a bit more to the side.
Now when you can do a safety, try doing either a stalefish or a blunt and try to tweak them more to the side each time you do them so you are making a straight air look decent.
Once you learn 3s off of jumps and you can do them with a decent amount of control I recommend you stop doing them immediately. If you have basic ability to spin then thats good enough but stop doing 3s because you will never progress by doing them over and over again until they look good. Eveybody has this problem of not making their 3s look completely pencil and retarded. The trick is adding a grab with your 3 so you look like you know what you are doing and not like you just came from ballet lessons. The main lesson is that you wont look any good until you start to incorporate grabs in your spins. Even this one kid at my hill who only focuses on style cant make his 3s not look pencil and gaper no matter how much steezy afterbang there is in his landing.
So the trick to this is doing a grab with a 180. If you fly past this step and try to grab with your 3s right away, you will fail. so start with a safety 180, learn to land switch. dont worry because it is natural to be scared of this so try to progress from small jumps on the side of runs to small park jumps. The fear factor comes from the speed you get on your landing so try to hit things a bit slower so you arent going switch all of a sudden with a bunch of speed. The way to get over this fear of landing switch with speed is just riding down the hill switch. If there is some space from the chairlift to/from the park then slow down and try riding switch.
The most difficult part is making a transition from riding on one edge to another so make sure you focus on this a lot as it is critical to your control and comfort riding switch. If you dont know what im talking about its basically this. Say youre riding from the right side of the hill (take the perspective of someone looking uphill from the bottom) to the left. Your right ski is below your left. The most difficult part is going from that to the opposite ( skiing leftside of the hill to the right side and your left ski is lower than your right). So make these transitions often so you have more control.
Once you get a safety 180 go to blunt or stale 180. once you get those try doing a 3 and pointing your toes down for a blunt/stale but just barely reaching back for the grab. this technique is key to getting comfortable in your new spinning position. each time you try, try to reach further back until you get the grab. Then the key is getting the grab earlier and holding on longer. At first go for only touching with the tip of your fingers, then go for something resembling more of a grab.
As far as boxes and rails go.... Try hitting a small short rail thats close to the ground. Get a feeling for sliding the rail by starting on one side and quickly riding over to another. Basically hope over the rail while still touching it a bit so you become more comfortable. keep hopping over and angle your skis further down the rail each time so you progress from just hopping over (skis are at 30* angle to lip) to hopping over more gradually (skis are 45* then 60* etc. to the lip). Each time you should hold contact with the rail just an inch longer.
EX: you start on right side of lip and then you jump over the rail to the left side of it. After a few times try to go further on the rail.