head unit installation - no car harness

[Pimpin Dsmstyle]

there was a deck in there. they taped the wires together with no harness' used.

Im probably getting an alpine 4 channel amp. I was advised to get one of the two certain models he suggested. I dont understand how to match ohms and wire things up correctly. I just hook wires to the amp until I like how it sounds. I understand positive and negative but bridging is a little beyond my understanding. Ohms too, I forgot about all of that shit.