Do you know the format? Two passes? How many falls, etc? Generally just what Joe said, start out comfy and easy to get the feel for the wake, because it will be different, smile and have fun. As you progress through each pass, just keep mixing it up, do some of your 180s if they are feeling good, because they are good for points. Try to do your biggest trick at the end of each pass, be it a nice big grab or whatever you've got. If you are still up riding and having fun, maybe throw your "hollywood" trick at the end (something cool looking that you've been working on, a big spin or flip or something, or maybe just try a really big grab). If you get a chance when the boat comes back around after a fall, or whenever you can, say something nice to the driver and to the judges in the boat, thank them for their time, because they are working hard, tell them if you like the speed, etc.
One more thing, this is your first contest, yes? You should be prepared for the possibility that you will get out there and crash on your very first jump. It happens a lot to new competitors, I've done it myself and my friends have too. If that happens, just laugh it off. When the boat comes back around just smile and try it again. But go easy on your first jump until you get a feel for the wake.
The main thing you should NOT do is act like a poor sport and take it all too seriously. And however the scoring comes out, be a good sport and show enthusiasm for the other riders. You may become friends with them one day.