Try out Coast to Coast - http://www.coastingafrica.com/ for some good backpacker info.
I wouldn't only focus on Cape Town, get out and about... check out the wonderful wild coast (Transkei), don't miss out on the Garden Route, see if you can get to the Drakensburg, St Lucia and KZN midlands, and obviously Kruger!
If you are on a budget but want to travel with like minded people try a Overland trip, there are loads of options - look up Nomad, Drifters, Sunways, and Africa Overland.
Baz Bus is the best way to get around, just from a conveniance point of view, as they have drop offs at certain Backpackers
Remember to bring sunscreen, it is going to be summer after all.
Anyway good luck and ejoy....