I'm upset with my friend and I hope he pays for it.
What made you so upset with him?
He stole my idea for a paper
That stinks that he stole from you. That is definitely not something a friend should ever do to someone they care about.
Shit! I realize my mistake now.
See as to why they needed to steal your idea, maybe there were underlying circumstances. Ask them to not do it again and tell them they will let others know if they don't listen.