How can i talk to my doctor about my depression?

I'm 20 years old and I've never talked to my doctor about depression--my mom's always been in the room with me and I've never felt comfortable. Now, though, she's scheduled me an appointment to talk about my insomnia and I'm finally going to get my chance to talk to him- I think. I figure that'll be one of the questions he asks when doing his assessment. The thing is..I just don't know how to talk about it. My mom has told me for so long that my depression is nonexistent that I don't feel comfortable talking about it. I always feel like I'm just whining or having "normal adolescent feelings". What do I SAY to him? I am so so scared. I'm afraid I'll chicken out when I get my big chance..