Find a local car club that is focussed on circuit racing and join up. There is a list of car clubs on the CAMS website (www.cams.com.au), so this might be a good place to start.
There are a few guys in the Classic Celica Group in Canberra who might be able to point you in the right direction too.
Best way to get started is to do super sprinting. You do four or five laps at a time, trying to improve your times, but not actualyl racing with the other cars on the track with you.All you need is a car in good mechincal order, a fire extinguisher, blue triangle indicating where your battery is and a secondary bonnet restraint.
Hill climbs have similar requirements and are also fun, but perhaps not as good value for money.