Question for Department of Social Welfare

Hello My name is Traci Green. I live in Victoria, Texas USA. I will be sponsoring a 15 year old child to come to Texas on the SEVP (Student and Exchange Visitor Program)He has been accepted to a SEVP certified private school here in Texas and we will be applying for his F-1 Visa. I was told by the Embassy in Accra that I will need to show paperwork that his family has appointed me his guardian while he is studying in the United States. Can you tell me how I can get this paperwork started? I will be arriving in Ghana on June 5th - June 13th and again on June 27th-July 7th.