What boat is it and where are you racing?
If it's laid back, bring a 6 or 12 pack of good canned beer. Go 18-24 if its a bigger boat.
Read as much as you can on racing before then, try this... RACING BASICS - Beginner's Racing Manual
Do your job for now, don't worry what goes on outside the boat.
Be agressive in what you do, give 100% all the time. Do not ask any questions after the 4 minute gun until you cross the line and things settle. Any other time, ask away.
Hike hard, like its your mission in life.
Crap lazy crew are easy to find and hard to get rid of. Most people rather sail short handed w/ good crew than w/ people that don't want to be there.
No matter what the breeze, where you are in the pack or who's racing w/ you, sail fast and hike hard. I've been on boats where no one is hiking and it's blowing 20+. All it takes is one to start it, others will follow. You can make up for lack of skill if you put forth great effort. You'll be invited back.
Pack all the regular junk you would for a daysail. A snack that won't melt in your pocket. PFD, maybe a small knife, spray top, etc. Nothing heavy and keep it in one bag w/ a biner on it to clip it to something so it stays where you put it.
There's a bunch more, but I need to bug out of work to go race my weds night series.
hike hard & sail fast!