Johannesburg has an enormous range of options for accommodation. The hotels in Rosebank -- the Monarch, Holiday Inn, Hyatt and the Crowne Plaza -- are all convenient to the shops and the Gautrain, but they are also expensive. There are many B&Bs in Melville; the Lucky Bean, the Melville Turret, the Wedgwood are all good value, very comfortable and they include breakfast.
Unless you are arriving after 8:30pm, we recommend that you use the Gautrain to travel between the airport and the city. It is very quick, clean, efficient and possibly the safest space in the country. You will need to buy a card, which you can do in the Gautrain station, and load it with at least R340 (the cost of a return ticket). The journey between the Airport and Rosebank normally takes about 30 minutes.
The Gautrain station is on the top floor on your right hand side of the airport Arrivals building. You will need to travel up two long escalators. It is signposted.
The train travels via two other stops to Sandton and it comes to a standstill where you must get out. At Sandton take the escalators up one flight and take the train to Park Station, usually from Platform B. If in doubt ask one of the many security officers or another traveller. On a weekday these trains run frequently.
If you don't have decent roaming it's probably a good idea to pick up a SIM card and some data when you arrive. You can do that from the Vodacom shop in the Arrivals Hall. It will cost you about R115 and include some airtime. Almost all South African shops and restaurants will use VISA and it's generally safe -- especially if you have pin and chip. There is 30 minutes of free wifi available from AlwaysOn at the airport. There are touts in the OR Tambo Airport arrivals area; they're not aggressive but you should not engage with them ...
Things to do. If you're arriving early or late you may be wondering about things to do. Johannesburg is a very interesting city, with much to do and to learn. There's a good list at https://www.inyourpocket.com/j