How to create a custom API for an application?

Question

API

Good hour. I have created a bunch of free ad sites on joomla + dJ-classifieds and an app for android devices. The app is pretty clumsy. A simple webview with nothing. But then I realized that I somehow need to make an API for my sites so that the application is even faster, etc.
Is there any ready solution for joomla or dj classifieds? plugin or something. I can't do it myself, and it will take too long to learn