How to learn enough C++ to write a pacman-style game

I would suggest that if you're looking at graphics.h, you have other problems to solve first - namely one of using a 20+ year old fossil compiler for an OS which has been dead for nearly as long.

So the first steps really are to
- get a compiler which is compatible with your actual operating system (tell us what you're using)
- learn some modern C++ using some appropriate books and online resources.

When you've done a few months of that, then we can talk about choosing graphics libraries and so forth.