Open iMovie and create a new project. If youre not familiar with iMovie, look to the left of the playback controls (play, rewind, etc) and you'll see a littler sideways slider with a pair of scissors on one end and a camera on the other. Slide the slider to the left, the camera side. If you dont have a camcorder connected to the computer, it will say on the screen 'no camera connected'. There should be a button right under this that says 'click here to use iSight instead'. Click that to show waht the iSight is seeing, and then you can press record to record the video.