Eat an early dinner the night before to ensure that it is digested enough in the morning so that you don't get any surprises during the race! You don't say if the race you're doing is just a HM or also has marathon runners, but many times HM races don't put port-o-potties on the course. Eat food that is easily digested - low fat, not a lot of whole grain carbs. Personally I have a lean protein (chicken tenderloin) with either white rice or spaghetti and cooked vegetables, and I make it a point to be done by 5:30 or 6:00 p.m. If I'm hungry when I go to bed I'll have a small bowl of cereal.
As Timothy said, don't skip breakfast and eat what has agreed with you in the past. Eat breakfast early enough that it doesn't sit like a stone in your stomach. Don't do anything new on race day. If they are offering Gatorade on the course and you have never had Gatorade while running, don't take it. Stick to water.
Pin your bib to your clothing the night before and lay out all of your clothes, including hat, sunglasses, socks, shoes, race belts, water bottles, fuel, hair ties, garmin, etc. or put the non-clothing items in your car. You should be able to roll out of bed and roll into your race day costume while half asleep and not forget anything.
It's going to be hard not to bolt with the crowd at race start; I handle this by not looking around and concentrating on my breathing for the first mile or so until I am in my zone.
Most of all enjoy every minute of your experience. If you run with music, try running without it and take in the sounds and feeling of the race from start to finish.
"God gave you your body as a gift, so you should take care of it." - My Mom