You can't. It's off and turns on automatically when a program trys to use the iSight camera.
When I try to open Photbooth I get a message that another application is using the camera.
Well, before you open Photobooth, is the green light on that's beside the iSight? If it's on, another program is already using the iSight camera and so Photobooth can't use it. Check your dock for icons with black triangles; often programs will sit running in the background with no windows open, so you won't even know that they're running. You must also stop any videochats you are participating in before you can use Photobooth.