Girls are tricky. I was one much worse than your daughter at that age, I felt like if I could tell my parents who they could be friends with, then they could tell me, but less that, I was going to do whatever I wanted to do and be friends with whoever I wanted. It was NEVER my friends that made me do things, I only did what I wanted to do, I was strong minded, as I still am.
Now, I think there's something parents forget about when it comes to protecting their child - teach your child to respect themself, to respect their body, and to respect NOT fear you. Your daughter is going to do all the things you don't want her to do and thensome.
Hormonal changes and a new sibling (even before they're born), a large homework load, large class sizes at school where there's a lack of personal attention - like - if you raise your hand in class because you don't understand something and by the time you're called on your question isn't relevant to what the teacher's going over now - or if you don't ask questions because you don't want the other kids to get annoyed because they're past you, if you don't get along well with and have respect for who's teaching you the material, or maybe you're above average in something and you're bored with the material, and having natural human curiousity, is enough to overwhelm your daughter...
And she's so young that she's living moment to moment and doesn't understand that what she does now could effect her future, and even if you tell her all the bad things that could happen, she's not going to take it to heart unless it happens to her or someone she knows. So I would say to do your best to get creative and educate her on everything she may come to face, like if she so desperately wants to use MySpace or AIM - have her write a paper on the pros and cons of teenage use of the internet, then in the conclusion have her argue why she should be able to use it - tell her if she can make a good arguement, maybe you'll let her use these things with some guidelines that don't totally invade her privacy but that let her know you will be keeping an eye on her, have her research sexual predators murders of teens that have occurred from these sites, if you do decide to let her have a myspace page - make her make it private where only people she adds as friends can view her page, tell her she can only have "friends" on her page that she already knows in person, let her know that without notice you're going to ask to see her page to check her friends list and the content of her page, don't pry too much but check her inbox and sent mail to make sure she's not talking to strangers, make sure she's there at the time and handle issues on the spot, etc. teach her to make educated decisions, but let her make choices for herself, if she does something you see as unacceptable, try to be understanding, put yourself where she is and really be careful how you talk to her about your feelings, and if you disagree on something ask her this question "why don't you trust my judgement?" - she could see you in two different ways depending on how you express your concern - a control freak that just doesn't understand her OR an understanding mom who's opinion & feelings she respects & trusts.
And about drugs and alcohol, have you heard of the A&E show Intervention? it's on at 10 on Friday nights, it's pretty graphic, but I think it could be a really good educational tool for both of you, you can see what leads people in and out of drug & alcohol use, and she can see just how easy it is to ruin your life from one bad decision, try engaging her in conversation about what's happening to the people and see how she feels, get her thinking about these things, hopefully she'll see how bad it could get, and that it happens to people from all walks of life, not just to people coming from bad homes, or dumb people, or poor people, etc., and in seeing this it will make her feel sad and scared that this happens to people and not want it to happen to her or anyone she knows, and in seeing that accepting help is never a bad thing - successful or not - she may feel more comfortable coming to you asking for help if she has an issue.
I've "talked you ear off" by now so I'm going to just say - try to be understanding, open-minded, accepting,and educating, and continue giving her your unconditional love, so that you become her rock, her strength and confidant, not her adversary.
Best of luck, hope this will help.