As Dooffle9 said, just book it and turn up. I did my first trackday a few weeks ago at Cadwell.
I was quite nervous and the 3 sighting laps seemed never ending. Thought I'd never learn the circuit, but you'll be amazed how quickly you do. Well, maybe not learrn it exactly, but you'll know what corner is coming next.
If you're worried about being slow or being stratled by faster bikes overtaking you, join the circuit at the back of the group. You'll have a ot of the session before the fasdter bikes lap you, unless Croft is a short circuit.
Worst thing is I've now got a long wait until I can do another.