very common question yo, probably gets asked every week...use search on this site or google..
small spoon..
you will need to attach power wire to 12 v under hood, fish it through into your cabin and to the trunk
...if you're lucky and your stock headunit has RCAs (or you got an aftermarket one with them) then connect separate amp to power + RCAs
...if no RCAs, you need to buy wiring harness to plug into stock harness and stock amp and to connect some output wires into your separate amp + power from 12 v (video below)
or if you have money to burn and you don't mind the sound of those oem audio plus systems, they are pretty much as plug and play as possible but IMO they are pretty overpriced and don't hit that hard.