You can have jump be your X button and your L or R button. Just set whichever shoulder button you don't use to shield with to jump, and only use it when you need to zap jump.
There's one thing above all others that you need to keep in mind when you're at the tourney - you're there for the experience and to have fun. Some of the seasoned players will be there for the money, but even they're going because they love smash. I would always get nervous at my first couple tourneys, and would always perform really badly, but once I learned to just chill, it was great. The smash community tends to be pretty friendly, so if you're waiting forever for your match, instead of getting all anxious about it, just sit back and strike up some conversation with the others in your pool or whatever. If a match isn't going exactly as you'd like, take it in stride, taking note of what's going wrong, and instead of panicking, just try to get some space and reform your game plan. If you lose, you lose. Encountering someone better than you isn't a bad thing. It just tells you that you need to work on some stuff. Don't forget to keep good sportsmanship before and after games, though. Basically, don't tense up for no reason, just take things as they come, and roll with it. I'm sure you'll have a great time. Good luck.
