How do I tell my mother that I cut myself?

I would first off list the problems and/or reasons behind your beginning to cut yourself, then end with "so I began to cut myself." Then you're going to have to let them know that you love them, and that you love yourself, too, and are trying to be stronger, so that's why you're letting them know. Then you're going to have to wait for this person to accept it. After that, you can start looking into resources together.

Before you tell your mother, though, I would look into what resources you can find on your own. There are a ton of resources through the internet in terms of support, so I would start there. Next, do you have (a) best friend(s), family member(s), or adult(s) you can trust and confide in? One of the best ways to help yourself is to set up a network of people that will love and support you as you try to get help with your condition. Also, you may want to talk with a trusted teacher/professor/school or university counselor and see if they have any resources they can point you towards. If you can build yourself a support network of others like you or who have experience in this field on a local level rather than just online, it will help immensely.

I acknowledge there was another answer posted here, but it was rude, offensive and unhelpful, and if you've already read it I'm sorry. Cutting is a psychological condition, just like depression or anxiety, and they can each lead into each other, so please, everyone, try and be more understanding of those around you with these circumstances.