well there isn't exactly a way to tell them, maybe if you tell them not to be mad (they will be pissed) then maybe they won't be too much and they won't send you away or something, when my mom found out I was crying I told her I wasn't well, I showed her my arms, ofcourse she was mad at what she saw (who wouldn't be right? your child is hurting themselves) but she wasn't so mad how she could be, before I had showed her, I had asked her/begged her or whatever , not to be mad at me, she was worried but stayed with me until I calmed down.
so there isn't exactly a way to tell her, but you'll know what to do because they're your parents, you know them, approach them the way you'll feel they'll push their madness aside and support you until you feel ready to tell them your reason and all the other stuff....
sorry i'm not much help with your question. and good luck hun!!!
life doesnt really suck.... guess i thought it did...... now I lost my friends..... because i scarred my whole life..