How do I validate my game?

[Insanest6849]

When I try to play it says could not validate counter strike. I have steam and I went to the manual and it tells me to go to the half life menu and click on custom game and there I should be able to activate it. But I don't know wher the half life menu is. I went to the half life menu after I started the game and there was nothing that said custom game. So I tried create game but there was nothing there that I could activate counter strike with. Can someone plz help me? Thanks in advance.

[RetepNamenots]

On steam, simply click on Games > Activate a product on Steam, then type in the serial number for the games

The counter-strike/half life manual is probably out of date now, as when it was originally released there was no steam, and everything was completely different. I remember those days!