Tutorials on using X window API are hard to find. You will have to literally wander through the man pages and the very few tutorials over the internet and the source code for many programs.
If you are planning on using GTK, qt etc you should check their respective websites. GTK has tutorials on how to use it on their website.
Well if you are planning on making an application using opengl, good books are available online while with SDL you can refer to their documentation.
: I haven't been able to find online tutorials for GUI programming with GCC under Mandrake Linux (or any Linux). I prefer C over C++.