Hello, you must create some steps in the database and others in the report:
1.- You must create a table that contains the user names and another table that will have the username associated with the fields you want to filter, for example region or country or something more specific.
In your report you must create a parameter, where you will use the default value formula = ENV ("username") and the result of that parameter is what you will put in your query in the where section, example where country.user = $ {username}