Have fun, dont expect to win, expect to learn. There is always gonna be someone in the room that can beat you, just as there is always someone in the room that you can beat. The wheelchair may only hold you back if you let it. I know a few guys in chairs who play very well. Most of the guys in the room will be more than helpful to help you with the rules of the tournament or to give pointers if you ask.
Enjoy the experience and play safe more than aggressive early on. You'll be surprised how that will get you ahead in the early weeks of playing.
Enjoy and have fun.