Guys I need an advice. I have a very good idea for a mobile app, but at the moment I know only C++. What would you recommend me to learn for building a mobile cross platform app, and, of course, what tools should I use? I'm completely lost, but I really want to develop this.

That really depends on the scope of the project. C# with Xamarin.iOS && Xamarin.Android for more platform specific needs (better hardware access and native feel) || Xamarin.Forms for a more simple app and easy cross platform UI development.

I would advise against Forms if possible. It becomes more of a pain the deeper you go.

Otherwise you can try React Native with JavaScript.

Or just start with iOS or Android first and use Swift and Kotlin and have a good experience.

Python and kivy. U will have to make minor adjustments to run it in Android or ios. Or else, most of the code is same.

@rik61072 what this guy said.

You should definitely check out Qt framework I love it and use it for cross platform Dev