The first President of the United Nations was Paul-Henri Spaak from Belgium. The post of President is separate from, and should not be confused with the post of Secretary-General.
The President of the UN is elected by the General Assembly annually and presides over sessions of the Assembly. Members of the United Nations Security Council (USA, UK, France, China & Russia) cannot hold the Presidency. Currently Vuk Jeremic of Serbia is the President of the United Nations.
In contrast the Secretary-General of the UN is the spokesman and leader of the whole organisation, not just the General Assembly. This position is superior to President. The current Secretary-General of the UN is South Korea's Ban Ki-Moon.
