How do I go about getting custody of my nephew

Unless you give the experts more information into the particular situation you won't get an accurate response.

Why do you feel the need to get custody? Tell us the whole story please.

ANY question on law needs to include your general locale as laws vary by area. Why do you think you need custody? What is the status of his parents? How old is he?

The only answer we can give with the scant amount of information you gave us is to consult with a Family Law attorney.