You learn computer programming. Then you learn Java. then you learn the Android environment (from the view of the software).
Then you get an idea for an app - that's the difficult part. What makes your app better than the 250 other apps that do exactly the same thing?
Then you develop it. Once it's done, you code it, test it, find the bugs, debug it, fix the bugs, test it again, find the bugs that you missed, or that you introduced by fixing the old ones, fix them, go round and round, until you feel that it's bug-free enough to release it.
(Since the first step takes about 2 years, start on that and get back to us when you can actually develop a non-trivial program.)