I have written a book that teaches you A LOT more about layers. Get it here: http://amzn.to/17bcto8
For more tutorials, check out http://www.gimpedtutorials.com
In this tutorial, I explain how to use gimp for beginners. This video explains how layers work, and goes over some basic selection methods that will be built on in later lessons.
This tutorial was made using Gimp 2.6. If you are running a different version some functions and methods, although slight, may be different. I recommend using the newest version of Gimp. For a tutorial on different Gimp installation options, including installing the newest version on a trial basis without making changes to your current installation, check out the tutorial below.