Where should I start with modding?

Hello, dear modders and forum users. I need help with where to start when modding unit and hero skills. I watched a tutorial video on YouTube, but it didn't have enough details. Please tell me in which folders the files for changing unit skills and hero skills are located. I would be grateful. I want to create my own small balance mod that will weaken overpowered units and strengthen obviously weak ones. In addition, it is important for me to strengthen currently unused skills; I'm not even sure about nerfs yet. I also want to adjust the skill wheel, so I would be grateful if you could tell me (possibly with screenshots) which files need to be changed. I think I can figure out the syntax.