"IF" you were only charged with underage drinking, you'll probably only have to pay a fine. How old are you now? They do not throw you in jail for that charge. You can also call and talk to the cops or the District Attorney ( the phone # should be on any paperwork they sent you ) and tell them just what you told me and ask them what you're options are. Tell them you're scared, a young mother or father (sorry!) and ask them what you need to do to take care of this?
Believe me, it's not as bad as you think.