1. Make sure your car is in shape and fit for the task at hand. Low fluid levels and knackered tyres aren't going to help you at all.
Go slow and take your time to learn how your car behaves as it's a different beast all together at track speeds.
Be aware of the cars that are around you and don't assume they will yield.
If they offer it spend a few quid on some laps with an instructor. They will show you the braking and turn in points as well as the best line to try and maintain.
Most of all, enjoy it