The best thing you can do for yourself at this point is to get help, because as much as no one wants to admit it, cutting is first and foremost a cry for help, an attention plea. Even if you never tell anyone, and hide it for years, the basic psychology behind it is almost always the same- a cry for help, a distress signal, a huge sign flashing this is not okay.
I cut for five years until I got into counselling. In that timespan, I took over 60 stitches (at different times, mind you) and was hospitalized for a 72-hour suicide watch once. There was talk of me signing in to a long-term facility to help me get things under control. It wasn't all just the cutting, of course, but that was what drew the most acute attention for me, probably because I have a blood condition that prevents me from clotting. But when doctors stopped talking to me about controlling my emotions and started talking to me about group homes and halfway houses for adults, that's when I realized everything had gotten out of control and I needed serious help. I left an abusive relationship, threw out my pills and razors, and got myself into counselling. All at once. So I know it's scary, it's really, really scary to confront something like that and even scarier to have to tell people (like parents), but trust me love, not doing it is scarier in the long run.
I've heard it said some people grow out of it, and some people are "faking", and all of those things that people say to tell you it's not important and please take your drama elsewhere. Don't listen to those people. What you are feeling is very real, very important, and it needs to be addressed. And sometimes, some people need other people to tell them these things before they can see the truth in that. People will say that it's "for attention" like that means it's not real, and they couldn't be more wrong. The fact that someone would go that far "for attention" is in itself an alarming situation, so that argument is invalid.
If you're underage, you will have to tell your parents. And that seems like something you cannot do, I understand that, but remember, you don't have to tell them alone. You have the right to have your best friend with you when you tell them. You have the right to arrange an appointment with you, your school's guidance counsellor or public health nurse, and your parents, and have the official help you tell them. You have the right to rehearse with the other person what you will say to them, and how you will handle their reaction, and you have the right to request that the meeting be terminated if they react angrily or abusive during the meeting. You can take control of this, and you have resources at your school, local hospital, and community to help you explain this and to help them understand in a way that won't make them angry. And remember, if they do react angrily, that there's a valid emotion behind their reaction- fear. They won't understand at first, why their little girl would do such a thing. It will scare them. So it's a really good idea to have a counsellor with you when you tell them, because that person can help them as much as they'll be helping you in that situation.
And other people have said this, but I'll say it, too: You can message me at any time. I'm on here every night, usually for hours, and I check in periodically throughout the day. Any message you send me, will be read and responded to as soon as humanly possible, but usually within a couple hours of sending it. I can help you calm down, or distract you, or give advice or just listen, whatever you need at that moment.