Track days are great! Not only do you get to learn how the can handles and how to handle it under pressure, bit it's fully legal and much much safer than hooning on public roads.
The requirements are very minimal depending on where you go and the event. My local track is at Mallala, about an hours drive north of Adelaide. For General Practice days, all you need is entry fee ($70), a full tank of petrol and a helmet which you can usually buy from Motor Bike accessory shops for around $50.
Some events also require a fire extinguisher and a secondary bonnet restraint.
I made my own restraint using a short length of chain, plastic tube and 2 clip things I bought from a boating shop. I bolted this to the front windscreen water bottle mount then use a tie down to hold it out the way when I am not on the track.
For the fire extinguisher, I bought a 1 Kg slim one from a Motor accessory shop for about $40 and bolted the bracket just underneath the read seat. The slim shape means it's not so much of a problem for rear passengers (though I haven't tested that yet) and the placement allows me to easily reach it while still strapped into the drivers seat.
The only other requirement for the car is a Blue Triangle sticker to indicate the location of the battery. Also available from Motor shops, and I have seen one that was just made using blue plastic sticky tape,
It's also suggested - if it isn't a requirement, to wear neck to wrist, to ankle cotton clothing in case of fire. At some events they also check that the battery if firmly located, the tyres have tread and the steering and brakes work as they should. As you will be braking a lot harder and more often than you do on the street, make sure you have a reasonable amount of brake pad left before you start. I had to change pads half way through a track day recently, not really fun when the callipers etc. are very hot.