(Android studio) How to get a user's id through his email in Realtime Database (Firebase)

In this case Thiago, you can use firebase authentication with some platform like facebook and google so you do not need a password, or you can include the password key in your db.

In order for you to search your db, it must be ordered, the firebase already gives you all these implementations, both "search" and "order", on the firebase itself it gives you the necessary examples to resolve your issue, follow the link . I hope I have helped.