This has a nice short video.
https://firebase.google.com/docs/android/setup

A nice Android guide.
https://firebase.google.com/docs/auth/android/start/

An overview for authentication.
https://firebase.google.com/docs/reference/android/com/google/firebase/auth/package-summary
Code examples for authentication.
https://github.com/firebase/quickstart-android/tree/master/auth

Overview for the database
https://firebase.google.com/docs/reference/android/com/google/firebase/database/package-summary
Code example for database
https://github.com/firebase/quickstart-android/tree/master/auth