To see if the problem is with Skype or your computer, I'd recommend opening up Photo Booth (you should find it in your Applications folder -- it's a standard Mac application). If you can take your picture with Photo Booth, your camera is working and your problem probably has to do with your Skype settings.
In Skype, go to the Preferences panel and click on the Video icon. "Camera" should be set to "Built-in iSight" and "Enable Skype Video" should be checked. You should be able to see yourself in the preview window. And when you're actually calling someone, make sure the video icon on the bottom of your window is lit up.
For what it's worth, I had a problem with the iSight not working on a Macbook Air. I forgot about it for a while because I didn't need to use it. But after backing up and then wiping the drive and reinstalling OS X for other reasons, I discovered that the iSight was working. So that might be something to try if your iSight doesn't work with any programs and nothing else seems to get it to work.