Can i get car audio help please!?

[lee21]

I want to get a good sound system for my car but i don't know where to start. I want like 12" subs in the back so im pretty sure i need a amp. But does the amp need to be hook to the car stereo? and can i use the factory stereo or do i need to get a new one? and should i change my speakers or just keep them? so can some one just tell me like what i all need to get subs in the back and what would be best? oh and my car is a 2007 pontiac g6 gt sedan..thanks