Create a profile for my Android app [on hold]

I made an Android app.

Now I want that every person that uses it, needs to create a personal profile. So I think that I need an online database and when people try to create a new profile the app checks if there is already the same email in the database.

My questions are:

• What do I need to make my database available online?

• Is it so simple to create a "profile network"?

• Will I be able to access the database's data whenever I want?

Thanks for the help!