Dear student,

I hope you are doing well, app development is quite an amazing idea u have there. If you want to development apps or anything and make your career in this  I can say the best way is by getting a mastery over computer knowledge directly jumping to app development can be little difficult so Join into a course of  computer science or software engineering. They will build a very good foundation for uh.

There are a few online courses and training like

Udacity.

edX.

Simplilearn.

Google developers training.

Coursera

They aren't for free but the offer good training for amateures.

Hope this helps!!