Well you need a 3G cellphone or a 3G datacard. Since you are looking into prepaid, obviously you'll need to buy this cash @ R2500+ .
To then go prepaid, you have a few options:
1. Via Vodacom = pay R2 per MB . You load airtime on your prepaid sim and data usage sumply subtracts from that.
2. Via MTN = buy monthly bundles i.e. 350MB @ R200 [which is much lower than R2 p/MB ] . All you need for this is a normal MTN prepaid sim and converting your airtime into bundles. You can also simply go at R2 p/MB if you don't buy bundles.
Vodacom got more 3G coverage, but do not have the prepaid bundles only the R2 p/MB option, MTN got both but lacks coverage.