First of all, just in case anyone is in doubt, your SA family will definitely need a US tourist visa to visit you in the US. The US has an agreement with some countries where their citizens can visit the US without a tourist visa, but South Africa is not on the list.
A good place for you to start if you want to help them, is to send your family a letter of invitation to visit you in the US.
Your family starts off by making an appointment for an interview for a non-immigrant visa (NIV). All the information for this process can be found here. They can also download a printable PDF guide.