Skills in an area like public health, nutrition, agriculture, nursing, engineering, economics, etc. provide the best pathway.
https://careers.un.org/lbw/home.aspx?viewtype=BG
http://www.coyotecommunications.com/stuff/workabroad.shtml
https://blogs.state.gov/stories/2014/10/30/working-united-nations-insider-application-tips