1. You need to download and install Steam http://steampowered.com/download/SteamInstall.msi
2. Go to your account or register a new one.
3. Next, you need to follow the link received after purchase, log in under your account Steam and take a gift to yourself in the library or add to your inventory.
4. After activating the game appear in the library or in your inventory where you can already start to play or activate later / give to a friend.