Well, the first place to start is to get the car well sorted mechanically then! Also, join the VW Club of SA, as Sides says. I'm in it, and they're a good bunch of people with a small but enthusiastic motorsport section, with CAMS affiliation. The place to go on the internet for info on grass-roots motorsport in SA is the Southern Districts Car Club, they own and operate the Lanac Park dirt circuit for Autocross and Khanacross, and their website keeps tabs on Motorkhanas and Khanacross, with downloadable supp regs and entry forms.
http://www.sdcc.asn.au/
If you want to know what's going on in the world of hillclimb then the Sporting Car Club of SA are the people to ask, as they own and operate Collingrove, which is the only permanent hillclimb course in SA as far as I know. I haven't yet been able to work out the cost of a normal hillclimb entry though. I have done a Come & Try hillclimb at Collingrove and would like to do more, but I'm a bit concerned that I wouldn't get many runs in per dollar. For people who regularly run hillclimbs, how much does a Collingrove hillclimb entry cost and how many runs do you usually get? I've heard that the State Championships are about $120 entry, so I'm assuming that the regular cost is somewhere around $60?