Wanting to get custody of my nephew

Hi ladies I will try to make this short. My brother had an almost 3 year old son with some girl. My brother is currently in rehab and my nephews mom is out there doing drugs and leaving my nephew with my dad. My younger brother (not my nephews dad) lives with my dad and he wants to try to get custody of my nephew because of his parents situation. We live in California and we want to know what’s the first step we should take to try to get custody of our nephew? We don’t want him to go into a strangers home so I know it will be tough. My younger brother has a clean record.