When you apply there, send everything you have. Every certificate you have about computere skills, language skills etc. Another very important thing is the picture. Do not forget it. And take a nice one. And try to find out the number and call them. One friend of mine worked for personell, and he told me, that he tried everything to get a job for every intern who called him and sounded nice to him. As far as I know, there are no interviews for interns at all. But maybe in some sections? I have never heard of any. People come from all over the world. The UN can´t invite all of them. So I think it is important to call, as personal contact is very important and they would never know, what you are like.