you just need a basic understanding of the power requirements of yor car and what class amp you need.
go to
http://en.wikipedia.org/wiki/Electronic_amplifier and read about the different amplifiers out there.
then when you understand that (or just pick up that class A is for audiophiles and Class D is for everyone else but still does a decent job) then...go look at your companies.
some companies make better products than others. I personally stay away from anything inside Wal*Mart and Best Buy when im buying an amplier, but if you dont care about sound quality, Best Buy just may work for you.
JL makes quality products
Rockford Fosgate make some quality products
Pioneer makes some decent sounding amplifiers
Kenwood i dont like as amplifiers, but they make some quality headunits
JBL i dont buy, but i have heard peoples systems with them, and it does sound good
Diamond i stay away from, but the D3 and D5 lines are good if you can find them
Pyle makes pyles of crap..i've heard stock stereos sound better in a 20 year old car
Blaupunct or however you spell it is trash in my opinion
Polk Audio was bought out maybe 5 or so years ago (if not more) and has been making trash in my opinion since then...even their new top of the line products
find a price point for what it is you want, take a look at the options you have, then go ask someone at a reputable audio install shop (not best buy or anything like that....im talking about real install shops like Audio Itch which we have here in Tampa). Ask them which they think is better, and they will most likely give you a straight up answer as well as try to sell you their stuff.
hopefully that helps you a bit...there is no end all be all with these things. if you need more assistance, look at my sig for a link to another thread i posted (i reposted from another forum and did not write the original). within that, you should be able to find a starting point.
good luck...
Rij