1) Learn Java.
2) Download the Java SDK for the android (see
here[
^] for a good start)
3) Design your application.
4) Code your application.
5) Test.
6 Repeat steps 3, 4, 5 and 6 until it works...
Or, try it, and see what happens.
Ask again when you meet a specific problem, rather than "How do I write a whole app?"