The UN Official Documents Database allows you to search the full text of documents issued by the UN since 1993, as well as scanned documents from 1946 to 1993. It includes resolutions of all of the UN’s principal organs, as well as Security Council documents and General Assembly Official Records.
UN Dag Hammarskjöld Library is the official library of the UN. If you cannot find what you are looking for from the UN Official Document System, try searching in the library under Research the UN. It contains UN Resources & Documents and Research Guides.
For further information on how to find UN documents, look at the United Nations page of the Law Library Research Guide.