The United States was a founding member of the United Nations, and was heavily involved in its planning and creation. The United Nations was a response to the devastation of World War 2, and thus was founded shortly after.
The United Nations was first convened on 25 April 1945 in San Francisco, and naturally the United States was present.
The United Nations was officially established on 24 October 1945, and the United States was a founding member, officially joining on the date of its creation.