You must apply directly to the missions since they know their staffing needs, and budget levels, and have contracting authority. Although not required, experience makes you more competitive. Development experience is usually gained by working in a developing nation or with economically-deprived communities in the United States. No, but a graduate degree makes you more competitive. For direct hire GS career employment, the minimum requirements are U.S. citizenship and age 16.
Minimum qualifications vary according to the position and are specified in the vacancy announcement.
FS direct hire career positions require you to be a U.S. citizen at the time of application and at a minimum 18 years old. In addition, candidates and their dependents must be in good health and pass a rigorous physical examination prior to appointment and candidates must receive a security clearance before entering on duty. Applications for foreign service positions are initially screened to ensure that they meet the basic qualifications for the specific vacancy announcement. Applications passing the first screening are rated and rank ordered, relative to the qualifications specified in the announcement, by a Technical Panel composed of subject matters experts. The highest ranking applicants are requested to come to Washington D.C. at their own expense for a structured interview.