Johannesburg's main airport is 'ORT', O.R. Tambo International Airport. The IATA code for ORT is JNB.
Jo'burg's secondary airport is Lanseria. Apparently, it's calling itself 'international', but I'm not quite sure there are actually international flights coming or going here. It is used by two budget carriers, though.
Between the two, there is no real alternative when coming from abroad. You will arrive at ORT. I've flown a dozen times or more to ORT and would say it's safe. Beware of taxi drivers, though. They are, like perhaps everywhere else, notorious for trying to pull a fast one.
Depending on where you need to go, your best bet is probably to take the Gautrain to either Johannesburg or Pretoria.