For those that don't know (I had to Google it myself):
In the United States, a Minor in Possession, or a MIP, (also referred to as a PAULA, Possession of Alcohol Under the Legal Age) is a misdemeanor. Anyone who is under the age of 21 and possesses alcohol in America, with the exception of special circumstances, is violating the law of the state. If sufficient evidence is shown to a police officer that the person under 21 has been drinking alcohol or is in possession of alcohol, an MIP may be issued to the underage drinker.
http://en.wikipedia.org/wiki/Minor_In_Possession
It's only a misdemeanor, and if it's your first time offense (I'm assuming it is), the punishment won't be too bad. Probably just the fine. Maybe some community service, but that depends on the judge.