You can already develop apps using Java and SQL. Rather than waste time stuck in the theory, get stuck in and find out what you don't know.
Ideally you find an app/problem that you personally would like to see solved, doesn't already exist and is simple (simplicity is important).
Some ideas to get started:
* A phone friendly calculator (whilst basic mathematics is not hard, think of it as a learning exercise for properly designing UI)
* A simple image editor (I believe it's still the case that the phone doesn't always take well colour adjusted photos and user adjustment would be nice)
* An alarm clock (think about a simple UI, setting timers, awaking the phone whilst in a 'sleep' mode, setting tones, etc)
Remember that there is a community to help, but also try to solve the problems yourself where possible.
If you can build these then I think you are in a good place to start working on more complex apps.
Ideally you find an app/problem that you personally would like to see solved, doesn't already exist and is simple (simplicity is important).
Some ideas to get started:
* A phone friendly calculator (whilst basic mathematics is not hard, think of it as a learning exercise for properly designing UI)
* A simple image editor (I believe it's still the case that the phone doesn't always take well colour adjusted photos and user adjustment would be nice)
* An alarm clock (think about a simple UI, setting timers, awaking the phone whilst in a 'sleep' mode, setting tones, etc)
Remember that there is a community to help, but also try to solve the problems yourself where possible.
If you can build these then I think you are in a good place to start working on more complex apps.