I would recommend researching the process and education that individuals pursue to become a Foreign Service Officer--here is the State Department link: http://careers.state.gov/work/opportunities that I found. Foreign Service Officers usually come from backgrounds in economics, political science, international affairs, and usually have foreign language skills as well. Many also attend graduate school before entering into the Foreign Service Officer training program--which also requires a lengthy testing and application process. From what I could read, less than 50 individuals are selected each year. I knew people in my graduate program who wanted to enter the foreign service, and it seemed like the most well-respected programs were places like Harvard, Columbia University (NYC) and Georgetown.
I hope this helps get you started in learning more!