Set up user with a Custom Role.
Create a custom Role without checking the all data column. You will find more info about Roles here- https://www.cloudonex.com/business-suite/roles
Create a user with that custom Role.
Now, when this user logged in will see the only data this user own. For example, in list customers or in list accounts, it will not show any data until created by this user. Admin also can set the owner when creating accounts or customers.