install both hammer and the game you wish to make a map for. play the game before trying to run hammer. run the SDK (not SDK base). select from the drop down menus at the bottom the engine o the game, then the game itself. Then double click 'hammer' in the menu above the drop down menus.
If then you get an error, you may need to verify game cache: In Steam: Right click the Game Click Properties Go to the "Local Files" tab Click "Verify Integrity of Game Cache" Do the same for source SDK and SDK base.