this is like saying "i'm going ot build a car, what should i put in it?"
you'll have to do much of this research on your own.
specific questions are more than welcome, but we can't make suggestions out of nowhere based on nothing!
component selection is based on numerous things in cluding what you're using the computer for, what you're willing to spend, what new technologies are/may become available, what specs you were looking to have, etc, etc...
someone who is a gamer is going to need a lot more oomph than say a grandma who just wants to learn to use email.
the reviews on teh sites listed are very helpful and i'm sure if you google it you can fine basic "how to build my own computer" starter tutorials.
barebones kits are a good way to start off...that way you know your basic components are compatible. check otu newegg's selection of barebones starter kits for more info.