How does a country become a Member of the United Nations?
Membership in the Organization, in accordance with the Charter of the United Nations, “is open to all peace-loving States that accept the obligations contained in the United Nations Charter and, in the judgment of the Organization, are able to carry out these obligations”. States are admitted to membership in the United Nations by decision of the General Assembly upon the recommendation of the Security Council.
How does a new State or Government obtain recognition by the United Nations?
The recognition of a new State or Government is an act that only other States and Governments may grant or withhold. It generally implies readiness to assume diplomatic relations. The United Nations is neither a State nor a Government, and therefore does not possess any authority to recognize either a State or a Government. As an organization of independent States, it may admit a new State to its membership or accept the credentials of the representatives of a new Government.
https://www.un.org/en/sections/member-states/about-un-membership/index.html