In order to adopt a child from a foreign country, the U.S. Citizenship and Immigration Services require that you must be a U.S. citizen, and if you're not married you have to be at least 25 years old. You should start the process by either choosing the country you want to adopt a child from, or start by choosing an agency, which might only deal with certain countries. You can join a support group of other parents who are also waiting to adopt a child from overseas. Be aware that adoption procedures may vary from country to country.
You'll want to choose a country that is a signatory to the Hague Convention on Protection and Co-operation in Respect of Intercountry Adoption, which was designed to protect children and families who want to go through an international adoption procedure while preventing kidnapping, buying or trafficking in children. In addition, the adoption agency you deal with should be accredited for Hague Convention adoptions and be able to walk you through the process, sharing with you whatever information you need to be eligible for adopting a child. You'll have to complete a home study, send in immigration forms, and arrange a dossier of documents including information about your family's finances, health status and character.
Once the agency matches you up with a child, you'll be sent a packet of information and photos of him or her. You can decide whether you feel you'll be able to meet the child's needs and if you think the child will fit in well with your family. Share the medical information (if any exists) about the child with your doctor and ask for his opinion. Often, you'll have to travel to the child's country of origin at least once to meet. The U.S. Citizenship and Immigration Services have to approve the adoption and the U.S. Department of State has to let the child's country know that you've been approved as adoptive parents. Finally, you'll want to get a Certificate of U.S. Citizenship and a social security number for your child.