You wanna get a head unit with a couple of outputs. One for a sub and one for front.
Then some components for the front (5.25"). JBL make some good stuff for little money.
Then you need a 2 channel amp to power the components (again, stick to JBL), or you could use a 4 channel amp. This involves using 2 channels for the front speakers and bridging the 2 rear channels to power a sub.