They're not going to just throw you, someone they don't know and probably assume has little or no experience (a fact that you should make sure they are aware of just in case), out onto a fast and dangerous racetrack and tell you to "go fast".

Odds are, they'll probably be walking you through some training throughout the day, starting with car control in an autocross environment, and working up to the full race track. And then when they finally do put you on the race track, you'll start out with an instructor present in the car at least until they're sure you're not going to crash their car and/or kill yourself.

Best advice, as others have said: be humble. Take any instruction offered. ASK for instruction. (Again, I expect that's going to be a big part of their plan, anyway) Never drive beyond your abilities. If you "win", great. If not, you'll still have FUN, and that's the point.