If the video footage that you want to edit is on a tape you'll need to hook your camera or tape player into your computer using a firewire cable. Plug in your camera's A/C adapter so it stays powered while importing. Remember to switch the camera into VTR or VCR mode. iMovie should recognize the camera, and the interface will switch from edit mode to camera mode.
Use the control buttons to advance or rewind the tape. Stop the tape a few seconds in advance of the scene you want to save. Press Play and then Import. Press Import again when you want to stop saving to the computer.
When importing you can leave your camera to run through the whole tape and capture everything, or you can control the camera and only capture the scenes you want to use. The first method makes importing simple, but uses a lot of hard drive space. The second method is more time intensive, but can make editing a breeze.
If I have the space, I like to save all of the footage. It gives me more choices when I'm editing, and it's easy to just delete the scenes I don't use.