Basically, DirectX and OpenGL use libraries similar to like Dark GDK use, but obviously to a more complex and detailed degree.
C++ will use terms like 'float' and 'int', just like Dark GDK uses to define float numbers ( eg. 0.01 ) or integer numbers ( 1, 2, 3 etc.) in conjunction with these to work properly.
If you've installed the dark gdk properly, go to All Programs on the Start Menu, and check out Microsoft DirectX SDK (August 2007), and check out the documentation there. It'll introduce you to the syntax's and prepare you for the kind of data input you'll need to learn.
Start with a C++ basics book, one that teaches you about GUI's, and see how you like that (not C++ for Dummies - it only teaches command-line programs).
I personally prefer C# to C++ as a language (C++ is VERY old...) but experiment and see how you get to grips with it all.
The very best of luck =)
G0DL355
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So Endeth What Was Begotten From That Moste Darkest and Forgotten Of Times...