What reports should my code generate ?

 
Hi Everyone,
 
I am building a shopping cart website which sells e-courses, and PDF documents.
For examples:
Customers purchase e-courses, then they can view the online e-courses contents, and then they can take a online Exam, if they pass, then my code generate a PDF Certificates for the customers.
Now on the Admin Site, there is a "Reports" tab.
What reports should I make my code to generate ?
For examples: My code should generate profit and loss report at the end of every month. What else reports should my code generates ?
Thank you,
May