just get a nice head unit with decent power, 50W X 4 would be nice, and high voltage output. and replace all stock speakers. anything is better than stock, unless your car comes with mark levinson system like lexus.
as villageideot111 said, "Whatever you do, don't get one of those stupid subwoofers in your trunk. They are freakin stupid, and sound terrible." subwoofer DOES make the sound much better, just dont over load your trunk with subs cuz that does sound terrible. I have one 10in sub and nice head unit with cheaper side 6.5 speakers and they sound nice.
Im no sound wiz but just adding my input.