I have a 14 year old son and I know that is different but we do have a few things that work for us. Communication is the key! We talk openly and very candidly. I have established a relationship where he tells me everything. I have always answered all of his questions about sex, drugs and alcohol honestly and without judgement from the time he started asking things. You need to establish a trust with her so that she trusts you enough to tell you what is going on in her life. If she knows she can come to you and tell you anything, she will. She needs to know that when she tells you things that you are not going to fly off the handle and ground her...which is hard not to do. I know I am lucky that I have a good kid...for now! I always play it as....if he tells me what happened and what went on, we discuss it like 2 adults and figure out what to do about the situation together. If I hear about it from someone else, which rarely happens, I treat him like a child that hid things from me...grounding and taking away privledges soon follows! We have discussed drinking, he has tried it, and no I don't condone that, but he knows he can call and get removed from the situation without worrying about the consequences. We talk about sex, I push abstinance, but gave him condoms...I know that it will ultimatley be his decision and if he chooses to go thru with a sex act, I want him protected. I was a teenage mother, I know what I did, so how can I expect him to not be human? She already has the values you have instilled in her, let her use them, but monitor her activities and ask lots of questions. Get involved...be the cool mom that drops them off at the mall and picks them up. Go with them a few times. Get to know her friends and make them feel like they can confide in you too, it helps, what she doesnt tell you, they will! Dont sweat the small stuff, but let her know what your limits are and that no matter what you will always be there for her looking out for her best interests. She will make mistakes along the way to being an adult...but those are the lessons that they remember! Just remember what it felt like to be a teenage girl and try to look at it from both views...I wish I could have had a mom that I could talk to the way my son and I talk, I would have probably done a lot of things differently had I been able to trust my parents with my secrets!
Take care and good luck!