How do I tell my parents?

Lately, i've been feeling really hopeless, and I want someone to help me through everything. I cut myself (not to kill myself, just to help my pain), I drink, I've started smoking weed, I've used 2 guys for sex, and I've been used by numerous guys. I don't know how to tell my parents that I need help, that I feel like I'm losing control. I just need someone to talk to and help me get through my problems. I know they're problems, and I want to stop. I can quit smoking, I could stop drinking, but the other things I don't know how to refrain from. How can I tell my parents that I want to talk to a therapist? And if I have to, how can I tell my parents about all that I've done?