Heh, ask your friends if they have any extra stuff. Seriously. I put a ok starter system togther for $50 + the amp wiring kit ($35, included power, remote, speaker wire, inline fuse block and main power). I got a Power Acousticks (spelling?) 400w amp and a pair of 10" Jensen (don't laugh) subs for $85 installed! Of course, I have gotten better subs since then (2 MTX Thunder 6000 12").
As for your setup, Nelson is right, if you find the right sales person (and they are hard to find, since alot of them make their money on commission, so they want to sell you everything), they will help you start with good speakers, then an amp for them, then subs, sub amp, line filters, etc.
A tip from someone who listens to a little of everything in his car: get a head unit with sub volume, a equalizer with sub volume, or an RCA volume adjuster (fits inbetween your head unit and the RCA's going to your amp) so you can turn the subs up and down at will. Sometimes, you want bass when listening to 50 Cent, somtimes you want nice rich sound, not scrambling your brain, when listening to some oldies
.
One more tip: Find a car audio dealer that is NOT Circuit City, Best Buy or any other huge name. Most of the time you will get a better deal, better service, and better equipment.
Good luck, let us know how it turns out!