By your question, it seems you are a newbie, I would say sorry if my understanding is not correct.
Please check below links that would help you a lot.
xamarin started with android 1
xamarin started with android 2
xamarin started with aandroid 3
Regarding database, source of data of an android app is not traditional SQL server or Oracle directly. Rather android apps have an in build sqllite database option otherwise it uses JSON data/XML data through web service. But by your question, I would say, try to get an hands on how an activity works, what is android app life cycle. You can pick up data handling afterwards.
Let me know if that helps.