Hi Nishi! Kudos to you for choosing a challenging subject and career!
Regarding a job in the UN , Check out their ads. For certain jobs there are country -wise quotas . So read well before applying.
2. As for mentor, my 25 year experience in banking tells me, there are coaches and there are mentors. Coaches give you work related tips, guidance ,they actually teach you. But not necessarily they are in a position to give you career push. That’s what mentors do. But we are not always lucky to get them. One has to be realistic about one’s expectations and clear about what to expect from whom.
Good luck to you!

Hi Nishi,
Congratulations on having such clear goals already.
One of our other experts will soon get back to you on steps on how to find a mentor.
In the meanwhile, I just wanted to let you know that we have seen your question and are connecting with our networks to see if we can find you someone who works in the UN and would be willing to take on a mentee. This may take a couple of days, so please be patient with us for awhile.