More can always be added, but this question has been asked and answered before:
"Why is George Washington considered to be the first president of the U.S. when it was really John Hanson who was elected first." answered by /u/lord_mayor_of_reddit
"Were there really other presidents before George Washington?" answered by /u/midwesternphotograph
"What's with the people who were "president" before George Washington?" answered by /u/RadomirPutnik
TL;DR: The first "President of the United States" under the U.S. Constitution was George Washington. Earlier "presidents" held a different office, with a different title, and with different (and more limited) powers under previous forms of American government. They were legislators closer to the Speaker of the House or President of the Senate rather than an actual executive. Their few duties made them only nominally more powerful than any other member of the Continental Congress. For the same reason most Speakers of the House or Presidents of the Senate are forgotten by the general public, so too are these early presiding officers of the early Congress.