How do you stop a young horse from YELLING in the show ring?

They usually do grow out of this. Maybe try doing some pretend shows to give her more experiences . ( Just do some schooling in a ring with a few other horses working in their as well ) also she might be trying to tell you she loves company. So try putting her outside in a paddock with lots of friends she gets along with, every day. Then she may grow out of it.