Some things to keep in mind....
It will take a few races to determine how your body responds to food/energy sources. In my first race, I started out eating way too much, to close to the start, and ended up (almost) hurling. Typically, you don't want to eat anything substantial closer than two hours before the start. Eat a good breakfast. This is what sets the tone for the day. I eat Belgian waffles smothered in honey with a banana cut up on top. Later, I'll eat a light snack about two hours before the race. Up until race time I usually sip on Cytomax and then I hit a Clif Shot right before the beginning of the race. I then hit a Clif Shot as needed throughout the race. Ultimately everybody is a little different so this may take some experimentation.
Hydrating is equally, if not more, important. Start taking in extra fluids a couple of days before the race. Pack plenty of fluids. Either by hydration pack or bottles. I prefer to use a minimalist race pack to store my water so I don't have to mess with bottles, I've seen too many ejected bottles lying on the course.
Have fun. Unless you're some kind of phenom, don't worry about your finish in the first couple of races, just finish. Just learn how to negotiate traffic properly and how to pace yourself. After you've got that figured out, go for it. Some people are naturals at this stuff. If you kill em' in your first race, by all means, move up a class. If not, don't feel bad about taking a few races (or even a season) in Beginner to learn the ropes.
Bear in mind that I'm no superstar by any stretch of the imagination, I'm merely a Sport Class bottom-feeder. I'm not one of the aforementioned "naturals", just a guy who has to work like hell just to flirt with the top-ten. I simply just love the race experience and comradery. Good luck!