Android basic course
The objective of this course is not to make a great application to get thousands of downloads, the purpose of this course is to be able to Learn, think of an application, know the steps, and start programming. Once we have a minimum idea you can increase the difficulty, polish things and as I said before, if you like it and are good at it, you could have one of the best applications that can exist, but to achieve it, you must first learn it.
That's why from free online training, I encourage you to take this course, and to look for as much information as possible about programming with Android, since in Europe it is the absolute leader. I'm sure you won't regret it.