I used to live in Streatham and commute into Oxford Circus, Soho and the City.
Your cheapest, easiest and fastest bet is to catch any one of many buses doing down the hill from right outside Streatham Hill Station station to Brixton.
Takes 10 to 15 minutes on a good day - the buses stop right outside Brixton Tube Station.
Take the tube into Victoria. About 20 minutes.
From there you can get anywhere on the Underground or use any of the hundreds of buses that stop right outside Victoria Station.
Or use Victoria cab rank if you are feeling flush.
"buses doing down the hill" in line two should have been "buses going" down the hill.
If you google in advanced search the words Streatham Hill Station map....you will get about ten google maps you can zoom into or out of.
This is one of the links that'll help you get your bus bearings.
In the same way, you can good the words Brixton Underground Station map....or Victoria Station map....
"you can good the words" should have been you can "google" the words.
Tilly the Temp Typist strikes again!