UN at a Glance provides a tidy introduction and provides links to the history of the organization, a list of member states, special observances, and more detailed information.
Introduction to and overview of the United Nations, provided by the UN's CyberSchoolBus site. This site was created for K12 students but can be appreciated for its basic language and relative brevity.
This page gives a sense of sequence - that is, the order in which things occur at the conference.
Keep in mind that beyond all the research and writing of position papers, you will be interacting with other delegates in person so you need to prepare for face-to-face discussions and public speaking. For that part of your participation, there are Rules of Procedure for the ways you interact with others.