How do I get a letter of legal guardianship?

I am a high school grad in CA & am going away to college in CO. I plan on living with with my brother off campus but the school says I have to live on campus since he is technically not my legal guardian. How can my parents and I go about making him my legal guardian? The school says there is just a form I need to send, but all the ones I've found online arent right. Any help is appreciated, I really can't afford that extra $10,000. Thanks :)