Lots of people think using a layer mask is only for the advanced Gimp users, fortunately this isn't the case at all. Let me explain you the basic usage of layer mask on a easy to follow step-by-step example.
Knowing how to select parts of your Gimp documents with the help of Quick Mask might be one of the first things you should learn when starting with Gimp. Let me introduce you to the basics of Quick Mask.