Do I have depression and how can I tell people if I do?

I've had periods of being really down over the past 2-3 years, but it's got a lot worse in the past 8 months.

I constantly feel like a failure, my grades have dropped, I have no motivation to do anything (homework or anything I used to really enjoy; I can barely concentrate on an hour long episode of my favourite show) I can't be bothered to eat, I'm tired all the time even if I get a full nights sleep and I pull out my hair.

I'm really worried and fed up of being this way, but I'm not sure if it is depression or something else. My mom and other members of my family had depression if that helps.

Also how on earth would I tell anyone about this? Won't they just think I'm making it up?