How do I tell my parents I want to become a costume designer

Maybe you should break it to them slowly by making costumes for your pets.

Yeah, I was thinking about making some costumes from some designs I made and actually make, and wear, them over the next 4 years.

sounds like a plan.

Hi iceburg.

As you're just starting high school, you'll likely have the opportunity to participate in community/school theatre by assisting with costuming. Show you're serious by volunteering in the area that interests you.

If you're in a city that's big enough to have a significant professional theatre community - ask to interview one of the costume designers as part of a school project. Learn more about their academic background.

I'd say you're a bit young to be making serious pronouncements about your career plans - test out design and costuming - see if you like what the nitty-gritty of the work is like. You don't have to commit to a career at this stage of your life.