How much does it cost to change your first name in Washington?

I'm 16 years old and my mom said she would sign papers so I can change my first name. I was wondering how much it would cost and how I can do it.. I don't have a drivers license or permit yet so that wouldn't have to be changed..How much does it cost to change your first name in Washington?Use the law of common usage, it's free

channel

structuretoptr