Hi,
You can start with the state dept. site on international adoptions. They can give you the basic info. on adopting from any other country. Here is the first part of what they have to say:
PLEASE NOTE: Under Guyanese law only Guyanese nationals, former Guyanese nationals or non-Guyanese domiciled in Guyana may adopt Guyanese children.
You can see it at
Intercountry Adoption Guyana
Hope that helps.