Help about getting custody?

Im 15 and my life sucks right now. my friends dad said hell take my mom to court for custody of me. I want to leave because my mom is retarded. Her step granddaughter pulled out a knife on me and started swinging it at me in my own house and her and her brothers and sisters lied about it and my mom slapped me across the face. this is one thing that happened but i really dont wanna put my whole life on yahoo. My mom took me off her insurance plan so i cant get sick or hurt because she cant "afford it" but she can go buy an rv for herself and her boyfriend. a nice new laptop,and a new convertible. she was never a real mom and she kicked my dad out when i was eight and im the only boy in the house. and her boyfriend is a ********. my friends dad is the only real parent figure i have. My friends dad can afford to have me and the only records he had he was found innocent. If my mom fights for me but i tell the judge i want to stay with my friends dad what are the chances he can get custody? I know at my age my word counts. any help?