There are a number of things you can do which will give you almost immediate results.
1. Do a freediving course with Trevor Hutton.
2. Train in a pool like De Jong in Pretoria with a buddy and practise exhale dives at 5 metres. CAUTION: You need someone to watch you, you need to build up gradually and you should do Trevor's course before you attempt these.
3. Join an underwater hockey club and start playing.
4. Perfect your technique. Again Trevor's course.
5. Get over to Miracle Waters, lots of spots at various depths down to 30 metres, on the way to Brits and practise with a buddy.
6. As already mentioned buy Umberto's book, one of the best.
As always Never dive alone.
If you are really serious about improving your diving PM me and I will give you Trevor's details and tell you about our group which trains at Ellis Park.