Leviticus 19
28Ye shall not make any cuttings in your flesh for the dead, nor print any marks upon you: I am the LORD.
This is the first bible verse that came to mind when I read this. There are more that I cannot think of off the top of my head. I am sure that others will post them as they read this thread.
Our body, as Christians, is the temple of the Holy Spirit. So harming yourself can be likened to harming the temple of God. We are not our creator, God is; so we are God's property and to harm ourself is to harm something that belongs to God.
Hope this helps. I am not a teen, so I really shouldn't be posting here, but I was led to after seeing your post in the main forum. Feel free to send me a message if you need somebody to talk to.