You should know how to do the following in C++:
1. Overload fuctions and Call Parent Methods (Class Structure)
2. How to use interfaces
3. Understand Data Structures and Custom Data Types
4. Geometry+ in math
5. Knowlege in 2d graphics and 3d graphics (especially triangles and rectangles)
6. Understanding Timers and Animation
These are just the basics. You can refer to the opengl.org documentation for more details on prequisites.