Follow below steps:
- Learn core Java first.
- Learn advanced java.
- Search in Youtube: "build complete application for android/IOS"(For IOS you need to learn objective C). You will get plenty of videos.
- Set up your environment according to the instructions from the video.
- Build the application described in the video.
- Then come up with an idea and try to implement that/include that in the same app or create a new app.
- Until and unless you will start to work on a project you won't get stuck and unless you get stuck you won't learn how to code an app.