Data modeling is to decide where and how are you going to store your data.
You need to think in the shape of data.
In how many tables/collections should you separate your data?
What are the fields of each table/collection?
How one table is related to another?
What are the data pattern acccess to your data? How do you retrieve data?
Tradeoffs of storing all data in the same database, in the same table, in different tables.