When citing references, you should always follow a certain style and it also applies to documents from the UN. There are many different techniques and they vary in what is required.
In general it is recommended that you write the document title and document symbol. When you quote include either page numbers or paragraphs. If you are quoting from the internet some styles require that the URL of the document is given, as well as date when the web page was viewed.
The UN Library in New York, Dag Hammarskjöld Library, recommends The Bluebook. It provides a guide to how to refer to different types of documents. The book is available at the Dag Hammarskjöld and the Law Library, Uppsala.
Here are two examples of how to cite the UN Charter.
Read more at How to Reference the UN Charter | eHow.
Refer to the UN Charter in APA format, Citing the Charter of the United Nations.
Here are some manuals on how to write litterature lists and cite references when using UN documents.