Everyone pretty much hit the main points.
As mentioned, figure out who you're going with, then just read their site to ensure you have the bike and rider requirements met.
After that, just show up with an open mind eager to learn, and the day will go smoothly.
If you're really nervous, I would also recommend attending a trackday first as a guest. (no charge).
We've had lots of people come up to watch first, and then quickly realize they can do this! Remember, it's just a nice, twisty road with cushions and all one way motorcycle traffic.
