I am depressed and dont know what to do?

Im turning 18 soon and I have finally been able to admit to my self that I am depressed. I don't know what to do any more, I am just so depressed and I want help but don't know how to get this help or know what I should do at all. I realized that I have been depressed for about a year now and the reasons are not very important, what is important is that I have finnaly achnawladged the fact that I am depressed and I have no idea how to handle this. I can't tell my parents it is hard to explain its just completely out of the question. Should I tell some one like a teacher? I cant tell my friends or parents so would maybe telling a teacher help, or would they not care? I just can't take it any more I have become a totally different person my grades are really slipping, I smoke cigarettes and pot almost every day, I was arrested recently for the first time for ualting someone on the street for bumping in to me, punched a bar tender in the face cause he kicked me out for screaming at him to sell me drinks, I got a 2 day in school suspension for telling my teacher "I will ******* smack you" because he grabbed me by my wrist when I was filling my cup at the water fountan and he thought i cut him in line, and a day after my 2 day suspension i got a weeks suspension for pushing one of the other students in my cl for old beef i had with him. I mite not even be able to graduate depending on my midterms. You get the point. I just cant take it any more I don't know what to do anymore. I can't tell my friends or parents that is 100 percent out of the question so PLEASE do not leave answers saying"Ohh just tell your parents" please don't there is just no way of that I can't explain why. Who should I look to for help and how can I turn my life around?