Java is one of those general ‘work horse’ languages that can kind of do anything which is great but can be overwhelming. I’d say maybe start with building a simple Android app to solve a problem you or a friend has, focussing on how the user experience works etc, and use any times you need a ‘backend’ for it as a chance to learn a bit of the web/API development part. I don’t really know Java so can’t assist with more specifics unfortunately... all the best with it!