Although track days are relatively cheap dont forget to factor in the cost of a couple of tanks of fuel, and when you're finished you're gonna need a new set of front brake pads and possibly discs and also a new set of front tyres (if your car is fwd that is).
As others have said, make sure your car is in good shape beforehand because it's gonna take an absolute pounding on the day.
The most important thing though is ENJOY IT! Track days are great fun. Oh and on the helmet issue - the track days I have done at Castle Combe, if you didn't have your own helmet you could just use one of theirs.