How do you know if you're suffering from depression?!

I really need to know, because I'm never really happy :/ only when I'm around people..which is like only in school kind of..but i feel fake when im acting all happy, and I break into tears quite frequently..and the fact that I lack social interaction has led to these thoughts that everyone is like plotting against me, and even a conversation with this voice inside my head. I really don't like talking about my problems with people..it makes me feel really whiny. But i still feel like I don't want to continue living this way if this is how my life is going to be. And I'm 13 years old & and female if that helps.