You can install a pre-built Report Template from the Report Library:

These Report Templates can be customized in My Templates (under Report Templates):

You can also create a new, blank Report in My Templates by clicking '+' in the upper right-hand corner:

When creating new, you will be asked to pick from one of the new Report Types. The description under each Type listed will explain its parameters:

Note: Reports will only show results from the Teams, Accounts, and Users that the current User is assigned to.

Customizing or Creating a template will bring you into the Template Editor. You can add Columns of data to appear by clicking '+ Add columns'

When adding a new column to your report, you can select a Type, Title, and Description.
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You can also pass this section and make your selections in the 'Data Fields' section. What you choose will update the information above automatically and you can customize your Title and Description from there:

You can also add Fields per Data Source by clicking on an existing Data Source, then selecting '+':

Then select several fields for that Data Source at once:

You may also add a 'New data source' to add rows to your current report. These will appear on the right to select from:

In the upper right-hand corner of the Template Editor, you will have options to Filter Account/Client/CoClient data, edit the report details, or Preview the report: