Once you get through reading some of the resources mentioned above, find a project to work on and just dive in. It's one thing to read about how to do something, and it's a whole other thing to implement them on your own.
If you don't have any ideas for your own apps, poke around GitHub, there are plenty of existing projects that would love to take on new developers and be happy to help walk you through getting started. I work heavily on
FRC Notebook (for Android) and
The Blue Alliance (which has apps for both Android and iOS) and would be more than happy to take some time and help get somebody acquainted with contributing to any of those projects. Shoot a PM my way if interested.