I suggest downloading and playing with Raylib[1]. It's written in C, not C++, and is a library for video games, but it can be used to produce apps too. It has a ton of examples for you to review and modify as you learn, and it is cross platform. There are even bindings to other languages if you decide to leave C and use your favorite language. It's a one-click download and setup. Couldn't be easier and more fun!

[1] https://www.raylib.com/