Years ago I taught myself to program PHP by focusing on both reading the PHP manual and simply starting to write. The goal was creating my own forum software (like everybody else was doing at the time) and by doing so, learning everything I needed about PHP (and MySQL for that matter).
Fast forward to now and I find myself in the same situation: I want to learn how to program for Android and I need a goal, or rather a means, to achieve that. I decided that (also like to many others are doing now) I want to write a kitchen timer.