Ask the company where you bought it.
how can i activate fritz12

In that case I think your best option is to use a free programm, like Arena for example.

Serial keys are once and done so even if you find a serial key it won't work. It's a little more work but there are free GUIs and engines. Math0t mentioned Arena; there's also Scid vs. PC and plenty of engines like Stockfish and Houdini 1.5a. Or if you have a little money, older versions of Fritz don't sell for very much money. Plenty of downloads right on site (more>downloads). Good luck!

Guys
These chessbase people have put in efforts to develop this software. If one just pirates this stuff, there wont be enough incentive to develop it further.
I can understand college kids have a problem paying for this but maybe you can go for an older version in case you dont want to buy.
Probably chessbase should make one of the older versions available for free download.
Sorry no personal offence intended.

Mr Rishi is saying you are meant to pay for Fritz 12. That's how you activate it, not by begging for serial numbers etc. I think you understand him fine.
You say you have no money, may I suggest stockfish as an excellent and legal alternative chess engine - which in fact has a higher rating than Fritz.
i have no serial key of fritz 12