If you are setting up your system for the first time, we recommend completing the Self-Led Learning Track: Owner's Reports & Dashboards. This course walks you through, step-by-step, how to Run Reports, change Date Ranges and gives a foundation to understanding Report Types and the four Salesforce Report Formats.
Below are the videos contained with the Owner's Reports & Dashboards Learning Track for quick reference.
See also: Introduction to Building Custom Reports & Dashboards
Change Date Range, Sort Order and Grouping:
Refreshing and Scheduling Dashboards:
Out of the Box Reports and Report Formulas:
Report Formats Deep Dive:
Report Types and Object Relationships