If you are ever at the crossroads where you have to choose which API to learn first DirectX or OpenGL, the opinion of the gaming community seems that you should go for OpenGL first; it is easy to learn,code is similar to C++ and moreover it is platform independent. Dont get me wrong: OpenGL is not better than DirectX.It all depends on the individuals choice.People are divided on this. many think that DirectX is better because it gives you more functions to work with than OpenGl and also it is very convenient to Program in the DirectX SDK. While other think that OpenGL is better because it gives you more control over your code.
what i sincerely think is that you should be comfortable with both the APIs.
here is a good site for OpenGL tutorials:
consists of complete guide to a beginner learning OpenGL wit resources such as source code and additional books.