Get some good tires, and have at it. Don't walk in thinking you will do awesome. You will most likely finish last for the first few races. My biggest help came from the people I raced against. I dunno how it is other places, but in the KY Region, everyone is really nice and always willing to give you tips. Just talk to people, especially those in your class. They will give you tips. Ask them what they do.
For example, I run STS. I was talking to this guy who had a 99 si racing in my class. He sayed his biggest improvement was learning slow in, fast out. Meaning slow down before the turn, then speed out of it. I tried it and it shaved a good 2 seconds off my time (and also his, he beat me :D). But yeah, I know I keep saying it, but ask around, especially ask the old guys who have been doing it for a while.
edit: I agree with Killer. Get a helmet. Ususally there aren't enough loaners to go around. I purchased an HJC CS-5 off www.helmetshop.com
, and it was like $70 in total shipped to my door. It is D.O.T legal and has a snell 2005 rating. Which I think is all you need for auto-x.