[HELP]How to get serial code for games on steam!

OK first off i made new account on steam and i backed up my old copy of mw2 and want to play on my new account but every time i try it it doesn't work bcuz i don't have the serial code or key-code for the mw2!! If any 1 can help me and tell me if there is a way to get the code and yes on my first account i bought the game for 60$. So please help me i would really appreciate it guys thanks!

ohh really well sorry then please close this/how do i play a game i bought on a different account and without doing any cd keys or w/e!