How to tell my best friend I am clinically depressed?

I'm 13 & I have this amazing friend who has been through everything with me. I want her to know that I am officially diagnosed as clinically depressed.. but I don't know how.. & what she would say.. I really want to tell her because she is a really great friend & right now, I really need a friend.. & if she knew, I'm sure she would understand more.. but how do I tell her..?