A single person did not truly “discover” the place that is now the USA, because Indigenous peoples had already lived across this land for tens of thousands of years before any Europeans arrived.

If the question is about the first known Europeans to reach North America, Norse (Viking) explorers led by Leif Erikson reached parts of what is now Canada around the year 1000, long before later explorers. Christopher Columbus never reached the territory of the present-day United States but his 1492 voyages to the Caribbean opened sustained contact between Europe and the Americas, which eventually led to European exploration and colonization of the regions that became the USA.