South Africa has two kinds of circle and the priority rule differs.
Roundabout (large traffic circle, usually with a raised island and a blue circular-arrows sign). You must yield to all traffic approaching from the right, which includes vehicles already circulating, unless a road sign or traffic officer instructs otherwise. Enter when there is a safe gap.
Mini-circle (a small painted or low domed circle at a suburban intersection, with yield lines on each approach). Priority is first come, first served: give way to any vehicle that will cross or has crossed its yield line before you. If two vehicles reach their yield lines at the same time, yield to the one on the right. Drive clockwise around but not over the painted circle.
In both cases traffic moves clockwise, keeping the circle on your right.
Signalling. Turning left (first exit): signal left on approach. Going straight: no signal on approach, signal left after passing the exit before yours. Turning right: signal right on approach, then signal left after you pass the exit before the one you want.
Watch for pedestrians at the exits and for cyclists on the circle, and never change lanes on a multi-lane roundabout without checking the blind spot.