Layer Mask Tutorial: http://gimpedtutorials.com/index.php/layer-masks-explained-lesson-5-how-to-use-gimp-for-beginners/
Layers Tutorial: http://gimpedtutorials.com/index.php/how-to-use-gimp-for-beginners-lesson-3-layers-and-selections/
Layer Modes Tutorial: http://gimpedtutorials.com/index.php/how-to-use-gimp-for-beginners-lesson-6-layer-modes/