You can go two ways on this (at least) and it depends on your basic skills and aptitude.
The first option is acquiring some hard skills. Engineering, nursing, IT, and teaching or training are good examples. An appropriate terminal degree, combined with a minor in a foreign language (Not French or Spanish unless you can become fluent. Turkish and Urdu are good choices for poor language learners as they are a little easier to learn and yet are exotic enough that no one expects fluency. If you’re good with languages, go with one of the major difficult ones – Russian, Arabic, or Mandarin Chinese) or international relations will open a lot of doors.
The second option is to study international development and/or its related disciplines. This will require a graduate degree and it covers a lot of different study options; I’d include international or public health, public policy, conflict studies, “development studies” and the big one, development economics. The trick to this path is that it can be very hard to go abroad with these kinds of degrees, because you’re not really doing anything a development project needs in the field. You run the risk of getting tracked into headquarters-based jobs – academia or think tank if you’re lucky, program backstopping if you’re not so lucky. The best way to mitigate that risk is to acquire whatever hard skills you can (grantwriting is a good one) while in school, and intern as much as humanly possible. Abroad if you can, of course.
Anyone want to talk about their own study and where it got them?