Did your daughter suddenly start dressing this way? Aren't you the one buying her clothing? My girl is 16 and sometimes she does "borrow" clothing from friends. And I'm straight with her and tell her she is not allowed to wear it. She might get upset at that very moment, but she gets over it.
You need to put your foot down now and explain to her that a woman projects the type of person she is by what she wears. I tell my girl, "If you dress slutty, that's the type of crowd you will attract and that's the image that boys will see in you...a sleaze." It might be a little harsh and she may not get it right away, but stick to your guns!
You don't have a "little about me" section so I don't know if you have any other children. Assuming she is your only child, you two must be close. It is so hard balancing that "mom" and "friend" thing with our teen girls. Be sure to let her know that she is the light of your life and you love her so much and that is why you care about how she portrays herself. Hope this helps you! Let us know how you do!