Want to get into more general Linux programming? The GTK library allows you to make games using the GTKGLArea, but also has a wide range of layouts, windows and widgets to make programs with user interfaces.
Want to get your toes wet in GTK? Start with the getting started guide. It starts with a few basic buttons and callbacks and builds a simple text editor by the end of the demo.
Where to go after "Getting Started"? The platform demo's page has a few very simple applications which covers things like how to make a simple image viewer or how to create and select a choice from a group of radio buttons.
Likely your new home. All of the functions, properties, and signals for all of the widgets are well defined in the documentation. You should be able to find what you need in there, and if you run into problems you can ask for help on the Gnome #newcomers IRC.