If the report you need is not already available, the 'Guided Reports' Wizard will walk you through creating a custom report.
Answer the 6 questions presented in order to create your report.
Step 1: Choose a Module to Report on
Step 2: Pick a Report Type
Step 3: Select Columns for Display
Step 4: Select Criteria to Limit
Step 5: Pick which columns to perform mathmatical functions on
Step 6: Choose how you want the report ordered
Custom reports can be edited in their entirety from the Saved Guided Reports page.
Can I have reports run on a schedule?
Yes, you can use the Task Scheduler tool.
Is there a repository for reports?
There is a page on the Koha Wiki: http://wiki.koha.org/doku.php?id=sql_library with reports written by Koha libraries around the world.
NEKLS also has a great page (http://www.nexpresslibrary.org/training/reports-training/) with a list of reports they have written.