I received an underage for alcohol consumption, what will happen to me?

If you go to trial, and lose, it is likely that the judge will sentence you to a penalty on the severe-end of the possible sentences.  Moreover, if you try to represent yourself, you will likely not be successful at trial.  Based upon the facts that you have provided, it is not clear to me that trial is, at this point, an unavoidable necessity.  In other words, it seems like it would still be possible for an attorney to step in and obtain a favorable resolution of this matter.  Therefore, I suggest that you consult with a local attorney as soon as possible -- your liberty is nothing to play around with.