So, after you have installed GTK+ there are a couple of things that can ease you into developing applications with it. There is the GTK+ Tutorial http://www.gtk.org/tutorial/, which is undergoing development. This will introduce you to writing applications using C.
The GTK+ Tutorial doesn't contain information on all of the widgets that are in GTK+. For example code on how to use the basics of all the GTK+ widgets you should look in the directory 'tests' (and associated source files) within the GTK+ distribution. Looking at these examples will give you a good grounding on what the widgets can do.