Try to find some records like birth, marriage, christening and death records and gravestones, which often show parents, spouses, children, years, or other useful information.
If you know where someone lived, you can try local church records there, many of which are on line, and also seek gravestone photos.
Do a search online for other Dreyer family trees at Family Tree Search (it is free) on line to see if there are common people with those you know. Use the advanced search option to specify some things if you get too many search results to go through.
Interview any elderly relatives you have to see if they know anything useful, also friends of deceased people who are still alive to interview, and check out what your other relatives know, ,cousins etc.
Check the egssa.org genealogical site (also free) for Dreyer name and see if they have any gravestones or family bibles that may be relevant and have more info.
Join the rootsweb email lists that are South African ones to see what has been discovered, and search their archives for Dreyer information.
If anyone was in a war you can look for war records ..start at egssa.org.
The idea is to start with what you have and try to grow it, by adding clues where you can, and one thing will lead to another.
I did not know my dad's father's name for example when I started, but now I can go back four of five generations on that line. I got my grandmother's death certificate which told me her husband's name. And that was the start.