great advice spongebob... here's my .02 on that...
the very first thing you should do is replace the stock radio with a radio that puts out some decent power. this is the cheapest move. because most stock speakers will sound alot better if you put some cleaner power to them.
this move alone will probably satisfy your needs
buy a couple year old pioneer 6200 reciever, you can probably get one online on ebay for 50 dollars. this would sound decent and have some bells and whistles.
id be careful, if you like loud music and you put a decent head unit in it, you may roast your speakers playing with your equalizer and bass boost.
if not change the speakers.
www.sonicelectronix.com (something like that) ive dealt with them for several things, good people.
if you have 4x6's dont spend more than $50 or 60 dollars at the very very very most
if you want components, buy a plate and some $75-100 5.25 inch components in the front
if still not satisfied, add a 4 channel amp.
a 4 channel 37.5 watt amp you could get for really cheap, use the crossover in it (probably goes to 200 hz) or buy some inline bass blockers for your front speakers ($6 online).
still want more, add another amp with a sub.
if your truck has 4x6's in the front dont spend more than $50 on a set, or you could buy an adapter plate ($10) and get some 5.25 inch components ($100)