To become a mobile app developer at school level, you should have the knowledge of python and java atleast.

You can also go through youtube tutorials.

Good luck..!

mit developed a website for aspirants like you

http://ai2.appinventor.mit.edu

Hello Prateek! Start by learning basic programming languages such as C, C++, PYTHON, Java, etc. After that you can do certain short term courses online or at coaching centers. All the best!