If you expect the Rand to weaken against say the USD then you buy the USDZAR future , and sell if you expect the Rand to strengthen, have a look at the futures presentation in the help section , quite a good presentation there.
My advice is to stay far away from the forex market. You have almost zero chance of making any money. That be the provence of banks who deal in trillions everyday. The odds are so stacked against you that you'll get mauled just on the huge spreads alone. Not to mention the massive moves. In this day with all these currency wars going on big moves will become even more common. Stick to the stock market.
bulls&bears is right - currency trading is best reserved for currency pairs that have high liquidity and small spreads (also don't do this on any platforms offered in South Africa - you need to take your money offshore first). Illiquid currencies like ours are going to be way to expensive to reliably trade. If you are looking to hedge, however, that is a different story.