Each track has its own rules so check on there website -
Normal -
Helmet (some tracks you can rent them)
Driving license (never been asked for it though)
Long sleeve shirt and trousers (or a race (romper) suit)
Insurance - Normal road insurance won't cover you, track day insurance is available (at a premium) but not (in my experience) normally required.
Usually you show up early, sign in have a noise test and a saftey brief.
Castle Combe has groups depending on experience and type of car.
Thats it go out have fun
Other Advice -
Drive fast
Don't Crash