How do i get primary custody of my precious baby boy?

i want to know how i would get primary custody of my baby. i have a really good job that pays me very well and i only work three days. i dont smoke or drink. my life is my baby boy. my ex doesnt work. we hav a domestic violence case pendin. he was just stop by the cops for drinkin and drivin. What do i do to get primary costudy and what are my chances?