Note taking app in godot what should i learn?

Godot could probably fit for what you are looking for, it can display those “mind maps” graphics, you can also use text and import pictures.

There are quite a lot of apps and tools developed with Godot that are not “games”:

Abount how to learn Godot, this question has been asked many times:

I would also suggest the official “Getting started” of Godot Docs:

You’ll probably want to start step by step, learning the basic concepts before going all out on making the full scope of your idea. Without any experience, creating that app could be quite a challenge for you.