how much money have you got?
A basic definition of an amp is a piece of equipment that enhances the output of your speakers (or subwoofer) ususally resulting in clearer sound at higher volumes than is possible off your headunit.
these two links might help you:
About amps:
https://www.fiatforum.com/ice-guides/43557-all-about-amps.html
Wiring an amp:
https://www.fiatforum.com/ice-guides/43556-how-wire-amp.html
A two channel amp can run two channels i.e. two speakers - front left and front right. A four channel could run four speakers e.g. front left, front right, back left and back right.
depending of the RMS per channel you can, on most four channel amp bridge them so it becomes effectively a two channel amp as the two channels combined together are more powerful. (Simple way of putting it, if you had 1 + 1 + 1 + 1 bridging it means it is 2 + 2)
what have you got so far if your car speaker and headunit wise?
