A tutorial for a program as complex and multi-featured as the GIMP is well beyond the scope of this presentation, and perhaps beyond most of our lifespans . Official documentation is on the scanty side, yet there is a large number of web sites devoted to the GIMP that can tell you how to achieve specific effects.
As the GIMP is largely modeled after Adobe Photoshop; manuals and tricks and tip's columns can easily be adapted to suit the GIMP. Some of the plug-ins go under different names; emboss under Photoshop is called relief under the GIMP. The GIMP's contrast-autostretch is equivalent to Photoshop's equalize function. Since the GIMP is completely customizable, all functions and menus can be set up to mimic Photoshop's conventions to ease this sort of translation of Photoshop's manuals. This is what I've done, and I highly recommend it. Even if you do already have Adobe photoshop, believe it or not, there is a large and growing number of features unique to the GIMP and it's not a bad idea perform some functions within the GIMP and others within Photoshop. configuring both programs to perform alike will only make switching back and forth that much easier for you.