I want to do a degree in international relations, can I get a job in UN with such a degree? Which job markets I'm I likely to work in with such a degree?

What careers are mostly in demand or needed in africa?

The following sites may be of help to you. I hope that they are.

Job Vacancies in United Nations and International Organizations

Youth at the United Nations: Jobs at the United Nations

International Civil Service Comission - Job Opportunities