Upon initially entering the report manager, you'll be presented with a history of reports that you've run, with the option to "Download" these or "Open" them where they have already been downloaded.
To create a new report click the Create Report button and you'll be presented with a list of available options:
Choose the report you'd like to run and you'll be presented with a screen similar to the following:
Format - The Format drop down allows you to select how you'd like to create the report with the options of either PDF or Excel.
Start Date / Finish Date - Most reports will ask you to specify Start / Finish Date Range. Select the range you require.
Once done click Finish and you'll see your report in the main Report Manager screen.
The Action column will change from "Queued" to "Executing" to "Download". Once set to Download, you'll be able to click on the link to open the report: