Get the cheapest head unit you can find that will navigate your music fairly easy and more importantly has enough preamp outs to hook up a 4 channel amp and a mono. If you don't have much money, you should buy good speakers and run them off the stock amp until you can afford to amp the speakers.
some people will put in a sub before they amp their good speakers. It's exciting to have that bass, but it will get old fast. you wont regret amping your speakers first. No matter what you do don't run a sub on stock speakers. It will sound like shit.
After that you can buy one or two subs. Depending on the music you listen to will help you decide your sub. I have had dual 10's, and a single 12 with a matching one that I put in when there was room. The 12's were awesome, but once I did some soundproofing I actually liked the 10's more.
Also soundproofing your car will make a HUGE difference, but only do this if you have a car that you plan to keep for awhile.
Finally. Make sure you get quick connections for your sub. I had a setup with a 2000w mono attached to the box of the one 12". Then I could take the whole thing out with just one plug. To add the second sub I had a quick connect on the speaker wires. It was awesome. This is of course if you have a car and plan to use the back/trunk or whatever.