The way I told my parents was first to let them in on the fact that I'm struggling, and that I want to find a way to get better. For me, I then went on to say that I'd been harming myself but that I really want help to stop. In my case I then told them what support I had managed to get out of the mental health team but in your case it sounds more like you need to tell them that you want their help in getting some proper help. If your harming isn't a sui attempt then tell them that. Explain why you hurt yourself - perhaps that it's how you cope because you just don't know how. And reassure them that it's not their fault because they will almost certainly ask themselves that.

Hope this helps.