What's should I learn to develop a gaming app in my phone

Answer 1

The answers shared about C & C++ are good.
Martin's answer is detailed.

But, you just want to get started, and develop on mobile no less! Which won't happen with C++ or C--unless, you count text/terminal apps as one...

I recommend html, css, javascript, then try canvas frameworks like p5.js

//If you search you can find many examples of game related code written on mobile & desktop alike. Good luck!

//example code
https://code.sololearn.com/WftFhf1E9UQI/

Answer 2

For mobile/android apps you should learn c++. You should also learn some c#