Firebase, storing users data

Hello i am creating a game where players can get coins from levels, also transfer coins to each other.
The problem I am having is that I do not fully understand what is the best option with Firebase to store users credits.
I got the firebase authentication to work and i can also change and update an existing field of a document in firebase, but I can’t figure what method should I use to create a document for every user and store every users coins, then update it.
I would appreciate any help!