go around and listen to some. you also have to consider what kind of amps you want to run. if you're going for an all-out system, make sure the head unit that you're getting has 3sets of preouts, that push atleast 4volts.

as for subs, how loud do u want to go? do u want to shatter windows or just have decent bass? ie in my accord, i have 2 alpine type-r's, with the alpine 1507 amp. it's about to shake apart now. in my civic, i have one jl10w0. its not amazingly loud, but it provides decent bass for everyday driving