Hi! I'm from Argentina and my english is very poor.
With a 12V power supply you can't get much power. The TDA2003 is a very common amplifier that uses 12V. This circuit provides 10W, I think but it has a moderated THD.
If you want more power, you can use the TDA1562. Is an IC designed for car audio. The datasheet says 55W but with 18V. With 12V it could give 40W. Note that te THD is very high if you want high power.
I hope you can understood me. If you don't, sorry, I'm learning english