How does one start working for the U.N.?

Applying for Jobs
The more jobs you apply for, the less time you have to spend on each application. The more jobs you apply for, the weaker each application. In addition, some UN agency HR offices black list frequent job seekers (someone who applies for most every job posting at a duty station). Only apply for jobs where you have a chance, where you meet at least most of the criteria for the job.
Make sure the cover letter does exactly what is asked for. Usually this means saying how your skills and experience match their requirements, and since each job has different requirements, it means EACH cover letter must be different. Keep the letter as short as possible and address the job requirements specifically. One page is nearly always enough for a cover letter. Avoid emotion in your cover letter; if you write something like “I want to use my skills to help people, to make the world a better place” I would not interview you. When you write a sentence think how it would be in the opposite. “I don’t want to use my skills to help people and I want to make the world a worse place.” If the opposite sounds silly, then you are not communicating well.

Do you know what your competencies - your core skills and attributes - are and how to sell them? Are you thinking about them and presenting them in a different way for each application, since each job is unique? What key words are you using to describe yourself and your skills?
Some people try to imply in their applications that because they want to work for an aid agency and because they care and have good hearts, they should be given a job. It really isn’t like that. As one of the authors of this document said, "If I had to choose between a person who cares passionately about poverty etc but is not focused and doesn’t present well and a person who can get a job done dispassionately, without being very concerned about he bigger picture I would nearly always choose the latter." Work hard on selling your skills and abilities, not your desire to help.
Look for public events in your immediate area where there is a potential for you to meet people who work in international aid and development. These can be conferences, workshops, lectures and open houses. Don't just walk in, hand around business cards and walk out. Talk to people, listen to the program, discuss with attendees what you are hearing and seeing, and be there primarily as a professional colleague, NOT as someone desperate to work.
When you come across a fantastic report or resource written by someone at the UN, find that person's email, write them and say, "Wow, that's a fantastic report, etc." That's a great way to meet people from the UN -- lauding them for their work. Be sincere. Write as a professional colleague; do NOT mention that you are looking for work, but feel free to encourage them to read your LinkedIn profile, your web site or blog to learn more about you.