Well, here are a few things I can think of that might help.
First, it would be a good idea to bring a bottle of water and something to snack on between rounds. This will help keep your energy up.
Actually playing in real life has a very different feel than on a computer; it may take a bit of practise for you to play to your "full" strength. Do you ever use an actual board to look at positions?
If so, then there shouldn't be too much of a problem with this.
If not, you may find yourself making blunders because you didn't "see" something - you should probably do a quick conscious blunder check before every move, even if you wouldn't normally in a particular position.
Always try to stay calm. If you feel yourself getting too excited, frustrated, etc., take a mental step back and try to look at the board with a new perspective to make sure you're making the best move.
You'll perform better if you are physically feeling good before your game. Take a moment and try to feel if you're too hot, have an ache, etc. and try to address it if you can.
You should probably read the full rules for whatever tournament you're entering (it might have its own rules, or it might be USCF, FIDE, etc. that you can find online). Pay attention to areas such as touching a piece, offering/claiming a draw, leaving the board, etc. This will hopefully avoid any potential problems that might arise.
As for preparing your actual chess game (keep in mind that at least for this tournament since you are facing weaker players, some of this may not be very important):
Decide on which openings you are going to use and make sure you're familiar with them, but be prepared for unexpected moves especially since you're in a lower rated section.
Make sure to keep practising tactics (which I'm sure you're doing!) and be confident that you know basic strategic/positional ideas (e.g. outposts, etc.).
Brush up on any basic endgames that you are not 100% confident that you can win/draw. For example, if you don't know them, it would be a good idea to look up the Lucena and Philidor positions.
Well, that's all I can think of right now, hope some of this is helpful.
