O.R. Tambo International is the closest airport. Many hotels can arrange shuttles to pick you up. The Gautrain can take you to Sandton, where you can switch to a bus that will take you into Johannesburg (but remember that Gautrain buses don’t operate on weekends). Be vigilant with taxi operators (as in any country); use only a licensed taxi with a working meter. If you’re renting a car, make sure you have an international driver’s license, and remember that here you drive on the left side of the road.
Johannesburg is a very big place, and to see everything in the city and on its outskirts—or even farther afield in the province of Gauteng⎯a car rental would be ideal. However, if you’re sticking to areas like Sandton in the north, you can get around via Gautrain buses and tuk-tuks or, in central Jozi, the Rea Vaya Bus Rapid Transit (BRT) System. Local mini-bus taxis shuttle just about everyone around to anywhere; they’re very cheap, but be warned that their drivers can be a bit reckless. You can travel between suburbs by car, taxi, or the Gautrain or Metro Rail trains.