It's a pretty simple process, once you get used to it.
What you need:
Source SDK (Go to the 'tools' section of your STEAM menu)
Hammer World Editor (
VERC · Valve Hammer Editor 3.4)
Download Source SDK and install Hammer. Once there, select the game you will be making maps on.
Then, double click on the Hammer Editor button.
After the Hammer logo disappears, you can start editing, a very good tutorial on using hammer can be found here:
FPSBANANA > CS:S - Counter-Strike: Source > Tutorials > Mapping > 101 About Source mapping
Happy Editing!