What should I learn?

I know the basics of glut.
I want to get more into this programming.
Should I do SDl or just do the opengl without any toolkit.
I looked it up and opengl is pretty hard. I use C++ to code. I am good with all the basic concepts.

If you’re still learning how OpenGL works, there’s no reason to stop using FreeGLUT. If you already have a firm grasp of OpenGL, then you can abandon GLUT for something else.

Be aware that there are two major “versions” of OpenGL. The fixed function pipeline (OpenGL 2.1), and the one where you do your own shaders (OpenGL 3+).

Most tutorials you will find on the Internet are for the fixed function pipeline, and it is easy to come to the conclusion that it is the correct way to go. But if you have anything more than just a basic ambition, I would certainly recommend to go the full way. As soon as you want to do some more interesting things, like lighting or need better performance, the fixed function pipeline will quickly reach its limits.

Learning OpenGL is quite a big step, before you can really do something.