Rather than targeting a specific organization, you would do better to think about the kind of work you are interested in and the skills required to be of value in that field. A degree in political science and international relations will not qualify you for any serious professional-level position an a major international organization like the UN, World Bank or IMF: More likely, you will end up in an internship somewhere (probably unpaid) to start with (but not in any of these three organizations which only consider graduate students). Once you figure out where exactly you want to be, you then will almost certainly need to go to graduate school for further training.
Of course, we don’t know what country you are from and we won’t know what qualifications you would need to get a job in the govt of your home country. You would do better to talk to someone there about how to get a job in the refugee services/immigration depts.