Five years after the events of Borderlands, a valuable mineral called "Eridium" starts flourishing through Pandora's crust. Handsome Jack, the leader of the Hyperion Corporation, secures this new resource and uses it to rule over Pandora with an iron fist. Meanwhile, rumors of an even larger Vault hidden on Pandora spread across the galaxy, drawing a new group of Vault Hunters to the planet in search of it.
Go to: http://store.steampowered.com/ and download STEAM client
Click "Install Steam" (from the upper right corner)
Install and start application, login with your Account name and Password (create one if you don't have).
Please follow these instructions to activate a new retail purchase on Steam:
Launch Steam and log into your Steam account.
Click the Games Menu.
Choose Activate a Product on Steam...
Follow the onscreen instructions to complete the process.
After successful code verification go to the "MY GAMES" tab and start downloading.