How to get Custody/Guardianship of Grandchild

We are looking for advice regarding Guardianship/custody to remove our Grandchild from a drug use/mental illness/domestic violence home environment. My 8 year old Grandson has only lived with our Son, his biological Father, for 18 months and it's an unhealthy environment. (prior to that he lived with his Maternal Grandmother since his Mother left when my Grandson was 19 months old). My Son and his girlfriend use drugs, (she) has a history of mental illness and they are violent with each other. My Grandson has not bonded with his biological Father and his Dad's girlfriend treats him pretty badly. We're concerned for his safety when he's in the cross fire of objects being thrown at each other and the violence.