Select your report type. Report Types are made up of the different Salesforce objects (i.e. Opportunities, Accounts, Leads, etc). The best practice for report types is to select the one that has the *least* number of objects based on the data you want.
For example, if I only want data that lives on an Opportunity, I am going to choose the Opportunity report type.
But if I want Opportunity fields *and* Account fields from the related account (like address) then you want an Opportunities with Account report.