First of all, congrats on deciding to take up triathlons! Some advice I have gathered from my parents last few years of triathlon competition (I've never done one since I am only 15):
Be sure to make a list the day before the race of everything that you will need for pre-race (like a jacket and pants if it starts early), during the race (obvious things like a bike, water bottles, watch, swim cap, wetsuit if you have one, helmet, biking and running shoes, etc., as well as stuff like food you can carry in the back pockets on your bike jersey to eat on the road and dry socks for after the swim), post-race (like food, sports drinks and water, and dry clothes), and during transitions (such as a bucket and towel to wash your feet after the swim, marker so you can find your things, small stool for changing shoes if you want, and maybe an encouraging sign or note to yourself, lol). This will help take away some of the stress of race day, and also allow you to add more items to the list as you remember them.
During the race, remember hydration and nutrition, especially if it is a longer tri. The bike leg is the most important in terms of food and water, so don't neglect it, and do some quick research to find out what to pack for the ride (if nothing else, some packets of energy goo, such as Gu or Hammer brand, and energy bars, such as Clif brand, as long as you try them before the race to ensure that you can stomach them). Don't forget to drink plenty of water for 1-2 days before and after the race to stay hydrated, and eat high-carb meals like pasta and plenty of bananas to avoid muscle cramps the day before the race.
Practice your transitions a few times, even if you feel weird doing it out of the context of the race. It will help immensely by giving you confidence that you can do it with speed and relative ease!
Other than that, have a great race and good luck! Enjoy it, and whenever you feel ready to stop, think of the great feeling of accomplishment (not to mention - most likely - a free massage and free food after the race!) you will have when you finish. Your first tri should be more about the experience and the completion than the competition, so enjoy it. Hope your training will pay off! =)