How I can create backups for my notes in Android app by using Gmail account or a server?

I have an Android app, and it's for writing and saves notes in SQLite database. I need to make backups for the notes by using Gmail account (If the user sign in with his email in the app, the app automatically syncs and gets the notes). Or by saving the notes on a server (If the user press the Sync button, the app automatically gets the notes (But here, how the app or the server knows if the user is the correct user, and gets his the correct notes?!)).

I searched about that, but I have no idea how I can start!

Can anyone explain the whole idea about that?! (If anyone can bring source codes or tutorials for that, it will be a good explanation! :D).

• Note: I am a beginner, so please try to make your answers more simple. Thanks a lot!

• Here is my app on Google Play Store: My Notes app