I don't know what you expect any of us to help you with. You haven't given nearly enough information to offer any but the most general sort of advice. The severity of the consequences depends greatly on the circumstances, such as whether the bridge your friends threw the rock off of crossed over an open highway, a river or what have you. The best advice I can give you is to tell your parents and take your lumps like a man. Also, stop posting about it on public message boards. As it is, should the police decide to persue charges, this thread is subject to discovery and can be used against you in court.