When reporting a server, please make sure the server is in violation of a rule that we enforce on player servers which you can find listed here.
You can read up and learn more about how to report a server to Minecraft for breaking their EULA by clicking here.
You can report a server by creating a Support ticket. Be sure to include the following information:
Your Player IGN
Evidence (Links or Screenshots)
Please be aware, creating fake reports on purpose to get another player punished will result in an instant ban.