start with door speakers see if thats good enough, there so cheap online it doesnt make sense not to swap them out ($50/pair for Polks).
Then if you want more loud or cleaner, your going to need a line out converter like the Audio Control unit which will allow you to tack on aftermarket amps and subs.
My whole system ended up costing me less than $500 and sounds unreal with the Shaker 500.
2011 V6 6MT, Kona Blue
, heated Saddle leather
***Procharged*** Mods: AIRAID CAI, Bama, Magnaflow Street CB, BBK S. Headers, BBK Catted X-Pipe, BBK 73TB, Strut Tower Brace, SR Springs 1.5, Koni STR.T Shocks/Struts, Bumpsteer Kit, Panhard Bar, LCAs, PowerStop Brake Kit, Black MGP Caliper Covers, GT500 Rear Diffuser, Roush Side Splitters, Stillen Front Chin, 30% Tint, LEDs, Black Super Snake Stripes, Polk/Alpine, Audio Control LC7i, RF 10" Sub, 19" Staggered AMRs