how2getgarrysmod
Go to steampowered.com, type "Garry's Mod" in the search box, click on the thingy that says Garry's Mod + (CSS, TF2, Orange Box, whatev), and click a button that says "Check Out" or something like that.
(EDIT: The reason I didn't include G-Mod standalone is that you can't buy it by itself, it has to come with a game that uses the Source Engine.)