Welcome, newbies! : ) Here's a photo guide to the mechanical assembly of a computer. I realize the hardware shown in this guide is obsolete now, but the core knowledge is the same, so roll with it.
What parts should I buy? Check out the AnandTech Forums' General Hardware section, where many people get advice on what parts to choose. You're totally welcome to join the Forums and post your own request for advice. Note the "sticky" threads at the top of the Forum to help you.
What tools will I need?
Who is this guide intended for? What am I expected to know? This guide is intended for people who know that computers are made up of individual components, and would be comfortable adding and removing parts from their existing computer. If you've upgraded a computer with a new add-in card or more memory, and installed Windows from scratch before, you should be ok.
However, I won't be held responsible if you buy stuff that doesn't work; or doesn't work together; or you assemble it and it self-destructs; or any other consequence. Build at your own risk.
I have just one advertisement, so pay attention! : ) RossMAN from the AnandTech Forums generously donates the Web hosting that brings you this guide, so here's a plug for his site, BetterCreditCard.com. If you've got a CC, maybe RossMAN has found one that can beat it. Maybe score a zero-interest balance transfer, and save interest charges for a while... : )