You can either:
Create a new report from scratch
Tweak an existing report and save it as a new report
The latter approach is easier, but it requires having an existing report that is close enough to the desired report that you have in mind.
Open the Explore page
Add desired fields, filters, sorting, subtotals, etc...
Click Save and provide a name for your report
From the Reports page, click the report you want to start from
Press the Edit button
Tweak the report fields, filters, sorting, subtotals as desired
In the menu, click Save as... and provide a name for your new report