Never provide alcohol to a minor or allow minors to drink on your property/event under your control.
Actively supervise all gatherings of youth. Be present and aware. Let other parents and adults know you expect the same from them.
Call your local law enforcement at their non-emergency number if you know of a party that might be happening or is happening. You could save a life. You can report parties without leaving your name and in any language.
Always call 911 in the case of an emergency, even if your actions contributed to underage drinking. Police will take your efforts to address the emergency into consideration when issuing a penalty.