How do I get custody or guardianship for my granddaughter?

My grandchild has been living with me from the day she was released from the hospital. She is now 6 in the 1st grade. Her mother rarely comes around and never calls about her. The last time and only time she was here was at Easter this year. She doesn't have a place of her own and cannot keep a job more than 5 months at a time. She drinks, party and does marijuana. What can I do to keep this child safe?