Select the
layer menu
Then, click on
add a new layer. (
http://docs.gimp.org/2.6/en/gimp-layer-menu.html)
Now check the background layer you've created inside the layer menu inside your gimp toolbox (the small box floating on your main window), it must be the last listed.
Another way to add a new layer is right click on the layer menu inside the toolbox and select
add a new layer.
You can find a good software guide on the gimp site -->
http://docs.gimp.org/2.6/en/ , but feel free to ask if you need more help.
