I would recommend learning DirectX 11. OpenGL is full of legacy code for backwards functionality. This means OpenGL will let you continue to do things the old way so you may be using functionally that has been outdated since the 90s.
Directx 11, on the other hand, has trimmed the fat and only offers an interface to graphics that is up to date. This will help you learn the more modern way of doing things. OpenGL will also have much of the same updated functionality but it is hard to know what is the state of the art in all of those API calls.
With modern graphics, there is a lot to take in. You will find you have to write a lot of code just to draw a single triangle. Don't be frustrated if you don't understand it all right away. Just try following some tutorials and get those working. Then start to play around with the tutorial code to get it to do what you want it to. You will learn as you experiment. If you have any questions or hit any roadblocks feel free to post your questions here.
Here are some good tutorials to get you started