Gautrain is a great service. You'll need to buy a Gold Card and load it. The fare to/from OR Tambo is about 130 Rand each way, the card costs 13 Rand, so to save time probably load one up with 300 Rand (Sandton-Rosewood is ~20 Rand if you want to do local hopping).

Trains are regular, about every 15-20 minutes.