There are some differences between OpenGL and WebGL. However, if you learn OpenGL, you will be able to pick up WebGL very, very quickly.

Most likely you will use either C, C++, Objective-C or Java while learning OpenGL. For you to work with WebGL you will need to learn Javascript.

For either OpenGL or WebGL, you will need to learn the OpenGL Shading Language (GLSL) as well.

So just pick one and have fun.