Hello there frequent flyers!

I've got a question: I plan to visit South Africa, during my vacation in Tanzania. I would preverably like to book my flight in advance (sorry if my english is incorrect, I'm Dutch ). The plan is to fly from Kilimanjaro Airport to Capetown (via Johannesburg?) and then from Capetown (via Johannesburg?) to Dar es Salaam about a week later. Can anyone tell me if it is possible to book these flights online before I actually go to Tanzania? If so, where??

Thanks a lot and remember to fasten your seatbelts! (Is it possible to make a more lame joke?? haha!)

Ielajs