Anything I need to do before I visit South Africa?

Someone told me I should take some immodium ad in case I get what is called travler’s diarrhea. I have my passport, but do I need a visa to get into South Africa? Also is there any shots I might need before I go? I had a friend who went to Ghana and she needed a yellow fever shot and a visa. I am going in November. Any other info would be great too.