Jurisdiction for child custody is wherever the child has lived for the past six months. If, however, you already have a court order, the court may have retained jurisdiction of the custody order if one of the parties still lives in that jurisdiction. If no party has lived in any jurisdiction for at least six months, you must look at the state that has the closest ties to the child and see if that Court will exercise jurisdiction. The reason a court exercises jurisdiction where the child resides is because that state and county will have the best available information regarding the child, including education, living conditions, etc. all of which are relevant in determining custody of the child. Within a state, you should file in the county where the child resides.