Don't fret or worry about it mate, you will get there and wonder at lunchtime what all the nerves were about, nobody will take the piss coz we all have to start at the beginning, go out with a view to take your time and build speed, DO NOT TRY TO GO TOO FAST TOO QUICKLY, do two full laps and get your tyres warm before getting up to your full speed, grab an instructor for your third or fourth session, don't do it at the start, if you leave it till later in the day you will have learnt the track a bit and will get more out of the instructor.
Drink plenty of water and don't eat a load of crap food, apart from that just get it booked and enjoy:biggrin