To Create a report based on field history tracking you must set field track history to the object. To know more about field history tracking setup Click How to Enable Field History Tracking in Salesforce
After enabling History Tracking, a button will appear in the fields section that will allow individual fields to be selected.
Report on Field History Tracking
A new Report Type is created when Field History Tracking is enabled for an object. Use this to see the field’s history of changes.
This Blog is very useful to learn about the field tracking history and reports.