Waking up in a seemingly empty laboratory, the player is made to complete various physics-based puzzle challenges through numerous test chambers in order to test out the new Aperture Science Handheld Portal Device, without an explanation as to how, why or by whom.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.