Clinical Depression

So.... yesterday in health class we watched a documentary on clinical depression, and I have been diagnosed with this.....(I guess you could call it a disease? eh i dont know) But I didnt want to watch this documentary in the first place but if I didnt i wouldnt be able to do the reflection and my grade would drop which cant happen because im in IB , so I watched it and I just got really sad . So what I want to know is how do I deal with clinical depression? All of a sudden I just got really sad and I started to cry . in the middle of the weight room in p.e today. I just started to think and I couldnt stop and there were people staring at me as I cried , and I tried to stop but couldnt , and I started to have these really violent thoughts about hurting all the people who were looking at me , and I feel really bad for thinking about them that way because none of them have necessarily hurt me. I just dont know what to do... I guess alot of you have gone through this and I wanted to know if you could maybe give me some advice , and I really know that I should seek some help , but I have and no one believes me because they think I just want attention because I hide it so well and most times it looks as if im happy but im not...