Who was the first president of the United States?

The first President of the United States was George Washington.  He served as General and Commander-in-Chief of the Continental Army during the American Revolutionary War.  He was elected to the Presidency by the Electoral College twice, in 1789 and 1792.  On April 30, 1789, he was officially sworn in to his first term as President.
It is important to note that, strictly speaking, there could be no "President of the United States" prior to the ratification of the U.S. Constitution in 1788, which supplanted the Articles of Confederation and created the "United States" as we understand the term today.  The Articles of Confederation represented a minimalist governing compact amongst the thirteen colonies in existence at the time, all of which were under British rule.
During the Revolutionary War, a national governing body - the Continental Congress - was convened in response to Britain's 'Intolerable Acts" toward the colonies.  The first President of the First Continental Congress was Peyton Randolph, elected by his fellow Congressional delegates in 1774.  Cyrus Griffin was the final President of the Continental Congress prior to the enactment of the Constitution and the election of George Washington as the true first "President of the United States."

The first president of the United States was George Washington. Washington was an extremely popular figure because of his character, reputation, and service in the American Revolutionary War. 
In the election of 1788, he was the unanimous first choice of all the members of the Electoral College. Since the country did not have political parties like we have today, there was no nominating process like we see in modern presidential elections. However, there were two general sides—the Federalist side and the Anti-Federalist side. Washington's sympathy was with the Federalist side, and over ninety percent of the electors selected via the popular vote were also Federalists.
Washington was elected again in 1792. Once again, he was the unanimous first choice of all the members of the Electoral College. By this time, the two major parties of the early American republic, the Federalist Party and the Democratic-Republican Party, had started to take form. However, there was still no party nominating process like we know today, and historians consider both sides to have "nominated" Washington for president. The Federalists, whose electors once again received over 90% of the popular vote, backed John Adams for vice-president. The Democratic-Republican electors backed New York's governor, George Clinton.
Washington refused to seek a third term. In his Farewell Address, he urged Americans not to be seduced by the factionalism of political parties. It was a warning his fellow citizens did not heed.
https://avalon.law.yale.edu/18th_century/washing.asp