Hammer editor
Using source SDK
Choose the game you want to make a map for (IE : CSS or TF2)
Then double click the Hammer editor and it should load up
If it is the first time using it you will get this;
Choose the engine ver
For CSS it will use Episode One engie
For TF2, DODS, Portal and HL2-EP2 select The Orange Box
Done.
Hope it hopes you