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.

Below are the minimum and recommended system specifications. However, you can use this link to check if the product is compatible with your setup.

System: Windows
Requirements:
OS: Microsoft Windows Vista (Windows XP is unsupported)
Processor: Dual-core CPU with SSE3 (Athlon 64 X2 4200 / Pentium D 3GHz)
Memory: 2GB System Memory
Hard Drive: 10GB of free drive space
Graphics: DX10 compatible graphics card with 256 MB of memory (Nvidia GeForce 8800 series/ ATI Radeon HD 2600 Pro)
Sound: 100% DirectX 10 compatible sound card
DirectX®: Microsoft DirectX 10