As a first step, define the Scope of Management. You should only be able to track the user login details for the users logging in from the computers that are within the defined scope. After adding the computers in SoM, you can enable User Logon Reports.
Select Admin -> Reports Settings -> User Logon Settings to open the report settings page.
Select the Enable User Logon Reports and specify the number of days the history has to be maintained.
Click Save Changes.