I'm going to start building my DJ Setup. How should I do it?

First off, I'm going to be using Virtual DJ. Great program, bought the
pro version, yum.

Now, I've got the money to buy decks, and the mixer, but I'm not quite
sure how to hook everything up. I remember watching a video about
someone using an external USB Sound card for a laptop to plug in a
mixer, but I want to use a mixer, AND decks. Is this possible? Any DJs
out there that can tell me?

Also, I have a Radioshack 4 channel mixer, what exactly can I do with
it? I'm not quite sure what a 4 channel mixer is, and what it does.
Can someone tell me what it does and give me some things I can do with
it?

Thanks!