Beth,
Yes, you can only edit a copied or new screen. You cannot alter our stock screens. So you'd have to copy the CustomerList screen and edit the copy. Then you can make your changes. I'd also suggest you watch the webinar on screens if you haven't already. In it they will talk about replacing tab links. You'll need to understand how to do that when your screen is ready to deploy.
To edit the screen, click Edit on the right side of the screen in Customize > Edit. You can also use the View to see how the screen looks.
- Adam