The United Nations has developed a site specifically for students. Don't be fooled by the title (it is not as grade 3 as it sounds): UN CYBERSCHOOLBUS has a good overview of the United Nations and its functions here
Below are some introductory videos that help to give an overview of the united nations. After watching them respond to the questions at the bottom of this post.
The Founding of the United Nations
A slide show on the Basic Facts of the United Nations
What were the motivations and events that helped to found the United Nations?
To what extent does the United Nations provide equality for all member nations?
What potential good could the United Nations accomplish in the world? Has it been able to meet that potential?
What potential problems might arise because of the establishment of the United Nations? In what ways have these problems been realized?