Just be honest with your parents. It was obviously an accident. Just don't try to hide it. It will just make it worse. Tell them about it before they find out by themselves and apologize. The fear only hurts a little I promise. You'll feel better after just coming out with it.
As a parent, I would have more respect for my child if they were honest and open about what happened as well as I would show more lenience if they came clean. It would make me proud to know my child did something wrong and on their own still showed the courage to do the right thing which is sometimes not easy.
After telling your folks you broke the plate, ask if it's a pattern which is still being made. If it is, find out how much it costs to buy a replacement, and then get the money together to do so. If you have to do chores or get a job to do it, do that. You'll prove to your parents you're a responsible and trustworthy individual.