Read ``Computer Graphics: Principles and Practice.'' It's an old book, but it is still relevant today. It will teach you the mathematics and fundamentals of computer graphics. It's a dry book, but no one promised it would be easy.
Once you're done that, you'll be ready to learn modern OpenGL (OpenGL 4.x or OpenGL ES 2.x) by reading the latest reference specifications directly.
You can find the PDFs at the Khronos Group website and at opengl.org.
That is how I have taught myself OpenGL. Books like ``Teach Yourself OpenGL in 21 Hours,'' or ``OpenGL for Game Programmers'' are naturally going to be out of date. Those types of books are for faggorts.
So be a man, take the difficult path, and you will be a cut above the rest.