The best way to learn anything (at least in software development) is to work on a project. This is how I learned android app development.
Here is my road map, which will guide you to build your own messaging app. I assume that you are an absolute beginner.
Learn Java or Kotlin:
The majority of Android apps are usually written in Kotlin or Java, learn any one of these. Master the language, solve some problems on leetcode, or codechef.
Resources: You can learn java from any of the tutorials from the youtube.
Once confident, install android studio.
Learn Android Development basics:
Learn some basic stuff like activities, activity cycles, jumping from one activity to another. For practice make a lot of small apps.
Resources: I found the CS193A course of Stanford for Android development as best. The course videos are available on youtube.
Learn Firebase:
This is the final step, Firebase provides you with a real-time online database and a lot of other cool features. This is the place you will store your messages, the user names, etc.
Resources: I found the firebase tutorial by TVAC studios as perfect for learning firebase, and simultaneously they take on a project for building a chatting app.
Finally having learned, firebase and Android you can build your own messaging app. Go get a play store development account and put it on play store.
