Users See Only Their Own Data
If you only want users to see data they are associated (connected) to or only data they have created, then this article shows you how to set that up: Show Records Connected to the Logged-in User.
That article covers how to set up user logins, how to connect the records created to the user who’s logged in, and how to create views that show that data.
Employees can only see the Tasks that have been assigned to them
Customers can only see the Orders they've created
Managers can only see the Time Cards for their direct reports
Users See Only Their Group's Data
If you only want users to see the data associated (connected) to their company, department, or other groups, then this article shows you how to set that up: Show Records Connected To The Logged-in User's Company Or Other Group.
That article covers how to connect users to their groups and how to create views that show only data for their own group.
Employees only see the projects assigned to their Department
Customers only see the job listings for their Company
Store Managers only see the sales for their Region