Translations:F.A.Q. - Frequently Asked Questions/37/en

Click on the 3-dot-menu besides the post and select "report".
You'll be asked some questions. E.g. you can select other posts by the user that violate rules.
You can and usually should forward the report to the instance of the author of the toot you are reporting.
Moderators and admins then can delete the post(s) and/or block the user and/or ignore the report.