When making reports, understanding the power of the BI tools is extremely important.
Before making a report, understanding the business requirements and the implementation of Sales Cloud is another thing to understand.
But when making a report, the first step is to choose a subject area. An area of withing sales cloud on which you want to report, a subset of data linked to a functional area that represents related data in a specific structure.
Understanding which subject area is best for your report, is something that comes with experience, unless you can find some documentation