How can I talk to my parents about depression?

Perhaps you could talk to a school counselor. It won't be made into a big deal, and be kept private, and they might be able to help you feel better. If you think you need medical attention, sit your mom down in a calm, adult like manner and say, mom, I need to talk to you about something serious. This is not me being a hormonal teenager, I think I may have a serious problem and I need you to take this seriously. I need your help. Then just go from there.