The United Nations officially began in B. 1945.
In 1st January 1942, the name "United Nations" was coined by then US President Franklin D. Roosevelt. It was first used in the Declaration by United Nations during WWII. It was when representative from 26 nations pledged their Government to fight together against the Axis Power.
In 1945, representatives for 50 countries met in the United Nations Conference on International Organization and drew up the United Nations Charter. The Charter was signed in 26th June 1945 by representative of 50 countries. Poland signed the charter at a later date and became one of the original 51 Member States.
The Charter was ratified and officially came into existence on 24th October 1945.