Where do you buy garry's mod?

1st. download steam (http://storefront.steampowered.com/download/SteamInstall.msi) 2nd. buy garrys mod at (http://store.steampowered.com/app/4000/) (you may need to donwload another game to make it work! i downloaded half life 2 deathmatch because its only 5 bucks)