I want to adopt my late brother's son who lives in Cameroon with my mother. His mother is deceased too. What should I do?

It may be possible to adopt your nephew in Cameroon, but this would be an international adoption. Although Cameroon has not signed the Hague Convention on Child Trafficking the United States has. As a result, you would need to work with an agency in the United States that is Hague Convention Certified. You can find a list of these agencies at:
http://adoption.state.gov/hague_convention/agency_accreditation/agency_s...

In addition, I would suggest that you visit this web site that is maintained by the US Dept. of Health and Human Services. It provides detailed information about Inter-country adoption from Hague Convention and Non-Hague Convention Countries: http://www.childwelfare.gov/pubs/factsheets/hague.cfm