I'm assuming you've downloaded and installed Steam.
1) Open Steam
2) Right click on the Steam icon in the system tray and click on Browse Games.
3) Then click the button that says "Register a product I already own..."
4) Scratch off the panel on the HL2 coupon and it should reveal a special product code
5) Enter that in including all the dashes
6) Click on Next and follow the on screen instructions
After it's all done, you should have the Counter-Strike: Source icon in your games list. Additionally, when Half-Life 2 is released, the game will be automatically be unlocked as you've already entered the product key.
Hope that helps, let me know if there's any problems