As far as I'm aware, WP8 only supports DirectX for graphics.
I personally haven't used OpenGL ES for mobile so I cannot say how different it is from a desktop version of OpenGL (may be the exact same with a settings change for all I know) but from what I've done with OpenGL and Direct3D before, they aren't that different really.. concepts are still the same, geometry itself doesn't change. There are tons of tutorials and example codes out there for DirectX, or if you have some money to spend, hit up a barnes and noble and pickup a book on DirectX programming (or see if there are any specific WP8 or W8 books out yet that might cover it).
Thanks for your interest in WP8 and bringing more functionality, apps, games, etc to the platform! :) Best of luck!
PS: Windows Phone Samples
This is the link of sample code from Microsoft that are C++ based. Most of the examples are C# based but since you said C, then C++ would be the best examples to look for. Some have Direct3D examples.