You have received solid advise. However, I would strongly disagree with lmmike on cleaning between relays. You will have so much going on since it will be your first time that you will not have time/need to run patches down your barrel between relays. I have shot entire weekends without cleaning and I shoot F-Open! Go and have fun, do not worry too much about your score but do the best you can. Remember to feast and forget. Celebrate a good shot (in your head), and forget the bad ones (also in your head), but sometimes Tourette syndrome seems to take over some shooters while on the line when they shoot an 8 because of a wind reversal! ;D
Get to the pits as soon as pits are open, take water, some snacks, eye and ear protection, and wear a baseball cap or a hat. Make sure you have plenty of supplies at your target, especially shot marker spindels. At 300 yards, shooters tend to shoot them very regularly. When you run the target up, make sure you put a paster sticker on your finger so you can have it ready when the target goes down. Move shot marker to new hole, paste old hole, check score marker to make sure it's in the right place, run target back up, get new paster on finger. Do this as fast as possible but also as smooth as possible. Do not try to do it so fast that you forget to move the score marker or you will catch yourself when the target is almost all the way up and you will pull it back down, breaking the shooters concentration, especially if he's a fast shooter.
Please do not paste your pasters anywhere on the target or target frame thinking you are preparing ahead of time. That sometimes becomes a distraction for the shooter and the pasters will not stick as good on the target when you need them. Plus, on the extreme case, you can void a National Record by making that target look different than the others if anyone points that out if a National Record was shot, and believe me, shooter will not be happy with you.
Remember, there are no stupid questions. If you don't know, ask. You will enjoy it and you will have a grea time.
Disclaimer:
Everyone on this site is only trying to help you, and since you asked we did our best to help you get through your first match. Therefore, we are not responsible for all the money you will spend after you shoot your first match and get hooked! ;D