If you bought the retail version, the cd-key should be on the back of the box/ manual/ etc.
If you bought it through Steam, it should be there iun your library. Right click on the game and you'll see an option for displaying the cd-key.
If you've downloaded the game illegally, you'll need to figure it out yourself as nobody here at Tom's will be giving you any help on pirated games. Its against the forum rules.