15 year old daughter socially isolated

My 15 year old daughter is at a mainstream school and has no friends. Her mental health is deteriorating and I’m very worried about her. Does anyone have any advice about how I can help her? I’ve found a few forums but there doesn’t seem to be anything that focuses on teen girls. She says she feels worthless and so needs to feel that someone her own age is interested in her and what she has to say.