Have look at wakefield park's website. Do a couple of these first.
http://wakefieldpark.com.au/index.php?option=com_content&task=view&id=26&Itemid=42
Licensing depends on what events you want to do. Most clubs run CAMS sanctioned events where you need a CAMS licence. Some clubs run under AASA permits at certain tracks such as Wakefield. I dont know anything about them as I've never done a AASA event but they are usually cheaper.
Speaking for CAMS events only:
If you mean circuit race = race on bitumen you have several choices.
Motorkhana where you are single car obstacle course type thing against the clock.
Hillclimbs where you are single car on a track with a start finish line against the clock.
Supersprints where you compete on a track in pairs. You'll get several goes of several laps with the fastest single lap time being the winner.
Circuit racing everyone would be aware of.
You will need to be a member of a CAMS affiliated club. There's plenty around. Club VW Sydney if you want to be vw faithful in Sydney. Check the club finder on cams website for others.
Motorkhana requires a Level 2 non speed licence. You just fill in the form and pay $57
Hillclimbs and supersprints require a level 2 speed licence. Same form but $103.
Circuit racing is different. I've never held one so I wont say categorically, but you probably need to do an observed licence test, may need a medical..
events you can run in... club vw supersprint in May, Huntley Hicllimbs at Dapto see
www.wscc.org.au