How do I "rein my daughter in"?

I don't know what to do with my teenage daughter! She yells about the most unimportant things, takes off with her boyfriend whenever she wants to, and curses and swears. Her grades have been slipping from A's and B's to D's and F's. As I was putting her clothes into her drawers, I noticed a box of condoms. When I confronted her about it, she admitted, after a long and tiring fight, she was having sex with her boyfriend but "it was her life" and "I don't have to snoop". I don't want her to end up pregnant and I want her to go to college to make something out of her life. Does anyone have any suggestions?