Don't even consider using the touts at the airport. I won't bother explaining why.
There is an official taxi desk but it is often unmanned. When you go outside there is a taxi rank and the official ORTIA taxi
O R Thambo International Airport Taxi Association are usually fine although of variable standard. Fare roughly R350 depending on traffic and routing.
I have often used and been pleased with the service offered by this company :
EZ shuttle is a provider of general shuttle services and airport transfers in South Africa.
The best option of all has already been given to you - the Gautrain!