Help?

So umm...basically my dad found out I'm cutting again. I don't know how; he must have been looing through my things again or something. I have to tell my mum when she gets home but I have no idea how do it. She's most likely to get angry and say that she's wasting her time with me and shit. I really need help because I have no idea how to say it to her and my dad won't do it cause he's to ashamed. Help.