OpenGL or DirectX are the APIs used to control the hardware, but 3D computer graphics is a pretty deep field before you even get into the engineering details.
I would definitely recommend you read Real-time Rendering, it’s the best and most accessible intro to game graphics and a great overview of the field.
As to OpenGL itself, you can’t go wrong with the OpenGL Super Bible.
One thing to be aware of is that any material on the internet that deals with OpenGl prior to version 3.0 is completely deprecated and not useful for modern graphics programming.
I would link you to some useful stuff on the internet, but for some godless reason I am not permitted to to so.