The hardest decision - do you tell others you cut, and risk them thinking you're a freak, treating you differently, maybe even spreading rumours? Or do you keep it inside?
If you can find some good person to talk to, someone you trust, tell them. If someone else knows, the pressure goes off a bit, its easier to resist temptation, you don't cut so much. The internet is excellent for this, you don't see the person, you don't know them, they'll never visit you, never meet your parents, never see your friends. They just know you, as you, and you don't need to hide.
But how do you tell someone you cut yourself?
Gently. Your friend is going to feel upset, maybe a bit disgusted (unless they've cut themselves), even guilty for not noticing. Go easy. You may be hurting, but they are too. Don't use it as a weapon. Don't go up to your mother and say "look what you made me do" - no one made you, you chose it. Don't use cutting to manipulate.
Reassure your friend. Tell them you're not trying to die. They're going to think you're trying to commit suicide - tell them you're not. Tell them you want help - if you don't want help what are you doing telling people? Whats the point? Tell them why you cut, tell them why you need it.
Be honest. If you trust someone enough to let them know you cut then you trust them enough to tell them whatever causes it for you.
Don't bug them. Let them have time to digest it. Its a big thing you just brought up - give them time to get their heads round it. No use losing a friend by freaking them out when time can bring them back to you.
Recommend some site to them - Secret Shame is excellent, or this if you like, some good psychological-type site to let them know about SI. They mightn't believe you - lets face it, you cut yourself, you're fucked up, and you've been lying to them about cutting in the past (overtly or not), its understandable they mightn't believe you. Recommend a site or a book or something, take printouts with you if you like, so they know what they're dealing with.
Don't just throw them in the deep end and then expect to be rescued yourself. You're not the only one affected by this.
If you want no one else to know, tell them. They might not be keen - they might not want to handle that by themselves. Find a helpline or something they can call, Samaritans or something. They need help too now. You involved them.
Afterward, if they accept it, if they try to help, don't go all freaky on them. Don't start cutting around them. Don't start showing them blades, don't start showing them dried blood on anything. Don't start reciting poetry about how you love your blade to them. They are not going to understand as another cutter will - of course not. Don't scare them.
Good luck. Let me know how it goes.