I hear this is a burning question for a lot of people, so I will attempt to provide some detail here with both a Mac and PC perspective. I'll go over a lot of things, seeing as people seem to want to know all about webcams!
Step 1: Get A Webcam
You will need, alas, a webcam if you are to partake in the festivities. If you have a Mac, you're in luck as long as it's not a Mac mini! You probably have one built-in. To check, click the apple in the top left of your screen. Click "About This Mac". Click "More Info...". In the bar on the left, click "USB". Now check to see if "Built-in iSight" is listed. Also, if you're using any non-built-in webcams, this is where they should show up. I've included a screenshot of Photo Booth, the bundled Mac program which came with your computer (if you have a Mac with a webcam), and a photo of the iSight camera on a MacBook.
If you have a Windows computer, you probably don't have a webcam built-in, and as such will have to buy one. What you are looking for is a "USB webcam". They're sold all over the place, and you get what you pay for. You can pick up a cheap Logitech for 25 dollars, and I wouldn't recommend spending more than 30 dollars until you know you're really a keen webcam person. Look for a high framerate (30 fps or more) and at least a 640x480 resolution. You may also want a built-in microphone, which is a pretty standard feature. (It's hard to find a webcam without a microphone, actually, but if you're buying a really cheap one, do check!)
It's worthwhile going to a large computer store (CompUSA, Compusmart, Future Shop, Best Buy, or whatever's large and near you) to have a look, as they tend to have one of each webcam set up and connected to a computer, so you can compare image quality. Some of the more expensive models do fancy things, like tracking (the camera follows you around the room) and have nice zooms, which may be useful, depending on your intended use.
I've included a photo of the Creative WebCam Vista Plus, which is a really cheap, very basic webcam, which is Mac and PC compatible. It's one I'd recommend for people just beginning to mess around with webcams, and you can find it and other Creative webcams for $15-20 USD if you shop around.
Now hook it up to your computer with the included instructions (most of them are plug-in-and-use now) and move on to step 2!