Welcome to Gaia! ::

First of all, quit freaking out, you're making yourself look like an idiot. If she doesn't want to stop, then there's not a whole lot that you can do. Just be a friend to her. Find out what's wrong. If it's something completely petty, then tell her she's being stupid and she needs to knock it off. If you're really good friends, I've found that when it hits far enough, ignoring that person does help...but I'm not suggesting it.
If it's really serious, drag in an adult.
Not much we can do for you, not much you can do.

thanks.... i hope that it works...

what works is tough love
like someone above said, if it comes to it, ignore em
cause sometimes people cut just for attention, and when you don't give them that attention, they stop...I dunno, it's worked for me...if its a silly reason, just tell her theres better ways to deal with it.

These are always tough situations for the friends of people who cut, but you also have to think about how tough the situation is for the person cutting. There is always a reason as to why someone cuts. And, as stupid as this may sound, attention is sometimes a legit reason. They feel as though they need to do something to get the attention of someone because something is going on and they can't deal and they need help.
Anyways, talk to your friend. Don't preach, just ask a few simple questions and then listen. If your friend is talking you don't say a single word. Wait until they're done for your questions and/or comments. If you keep interrupting you'll get less and less information.

The key to this is being understanding. If you keep going on about how you can't understand why they're doing this to themselves and all that good stuff they won't listen to a word you have to say because they know that you don't understand and won't want to waste their time with someone they can't have an honest conversation with. It's tricky, but if you want to help, you've got to try.

You can always PM me if you need any help or advice. ^_^