How Should I tell my parents that I'm depressed?

For the past few weeks, I’ve been feeling down and unable to do things and gain joy from things that usually make me happy. I’ve been getting barely any sleep. We just got our report cards in school, and the way that I have been feeling has made my grades suffer. I need a way to tell my parents how I’m feeling in a way that they would understand what’s going on and could get me help.