Way simple... you're a switcher, right? (thread) I've found that sometimes switchers can make things tougher than they are -- it's almost that you don't expect it to be so intuitive. Don't worry, very quickly you'll be "thinking" like your mac.
In iChat, you need to make sure the camera is enabled (video pull down) as well as your microphone, and you can double check them in a video preview (all same pull down) -- there you can check the volume levels and if you need to do more selections, you just do those in prefs (that button is on the vid preview window). When everything is set up right, your buddies will see a green microphone next to your name (you will too) when audio is ok, and a green camera means a video signal is available. They can also right click on you and the available options will show, including one-way if one or the other is only so equipped. It is a little weird doing that (being on camera when they aren't), but it's good to make a point (like go buy a web cam).
For iMovie, once you have the feed going in, there's a "Camera Capture" button on the left about midway -- iMovie help will show it to you with details how to use it, but basically a big window opens, you set the destination for the new file, click OK to start recording and stop to, uh, stop. You don't need the tape since the recording is going straight to the drive.
That should put you in business!
GB