Talk about it. In an emergency, call 9-1-1. If it hasn't reached that point yet, talk to her about her depression and ask her if she has thoughts of suicide. Tell her to call her doctor right away and get the help she needs. If she doesn't get help and you're still worried, talk to a trusted family member. If no one is available, talk to a teacher, or school counselor. Again, when in doubt call 9-1-1.
Let someone know. If your mother is saying she wants to hurt herself or is very depressed, please call a close family member right away. Ask them to help you get your mom to a doctor as soon as possible--even if it's the er. I was in a similiar situation with my mother and i know how scary it is. She may say that she's fine, but explain how worried you are and that you want to make sure everything is ok.