I assume you have the factory stereo/HU. In the w/o premium sound models (you'll find "stereo" printed on the speaker grills in the front doors) your deck has a built in amp. There is no pre-out that stereo, the signal is amplified (a tiny bit) and then sent on to the door speakers. I highly highly recommend that if you don't have an aftermarket HU, you buy one with your speakers first. Then if you want to add a separate amp later, you can look around. From what I have read here and my personal knowledge, I can highly recommend JBL. They seem to have a great power/$$ ratio.
In terms of buying and powering your speakers, I suggest you only buy speakers for your front doors. The rears do almost nothing for your sound stage, and you shouldn't waste a lot of money on them. Besides, no one can fit in the back anyway. Similarly, don't bother amping the rears, just buy a 2/1 amp and use them for the front speakers. It will also save you money with the amp (not just the speakers) by leaving out the rears.
Hope that's enough to get you started.