With aftermarket head unit, you use the RCA outputs to provide the signal to the external amp. There's also a 12V turn on lead that you usually route along with your RCA's (it's usually a blue wire that comes off your head unit). For subs, just get a class D mono amp. Do you know about what you want to get?
2013 Race Red GT/CS 6MT; Elec Pkg; RAAMat on the doors; Morel Tempo 5x7 coaxials; GT500 axle-backs painted black