Be prepared for anything- including the possibility that she may be a completely charming teenager who understands some things a good deal better than you do. 
Just concentrate on being the best, calmest, most decent and most impressive adult you can.
And remember that the end goal is not to bring her back under your control: it is to help her become an independent adult.
Let it be known that you are unshockable: that there is nothing so bad that she can't tell you, either concerning herself or a friend.
Also let her know that she can always rely on you to help her out of a bad situation, either by taking on the part of an ogre ("I'd love to come for a sleepover with you and your creepy boyfriend but my mum just won't let me") or by turning up in the middle of the night to pick her up, drunk and incoherent, from a party that is going wrong.
Let her know that you will always want her to be safe more than you will want to be spared the gory details.
Find something you both enjoy doing together, however little. When the going is tough, you need some normality, you need some fun together.
Never underestimate the power of a shared laugh to get you through even the darkest times.