J,
you need helmet, clothing that covers your whole body, decent shoes (not like boots and stuff), and a car that doesnt leak fluids and one that has good brakes - sandown is a heavy braking circuit, so you need to make sure that brakes are a1. Probably best to take the car for a service a few days before to change brake pads, oil all that - took mine to revzone in ringwood and told them to put the good stuff in.....and the car lasted the whole day. Then check the car by driving it around, check underneath that nothign is lose, and check that all is in working order.
Possibly good tyres as well - can get a bit scary in the wet (but a lot of fun - just go with the instructor for a few laps). Also make sure there is nothing lying around that will fly around in your cabin (eg paper, bottles etc).
Fire extinguisher is not needed, but can come in handy if anything does go wrong.
The point of the day is to learn how to manouver your car aorund the track, get to know the car, and just enjoy - not to actually race