Look - if you go back far enough in history then everyone is an immigrant to everywhere. It depends on where you want to stop your search. Who discovered America cannot be answered, unless you define what time period to use, and qualify what "discovered" entails.
Who came to the Americas over a "land bridge" way before European explorers? The people you call Native Americans.
Who likely landed in the Americas around 1000AD? Leif Erikson
Who discovered America for the Europeans during their period of major exploration in the 1400s, but thought he had landed in the East Indies? Christopher Columbus. This is why Native Americans are sometimes called Indians. He thought he was in East India.
Who discovered America wasn't the East Indies, but was another place, shortly after Columbus? Amerigo Vespucci.
There may have been people before, after, and in between all of those people. There is no way to know without a full recorded history of every travel, or full archaeological evidence. There is likely only one place in the entire world where people are "native" to a place, and that is wherever the first humans began - via creation, evolution over time, or whatever really happened or you choose to believe. Every other place in the world was settled, resettled, warred over, and settled again via travelers, nomads, tribes, nations, and armies.
Otherwise, people native to a place are those born in a place, regardless of how long ago their ancestors arrived in said place.
You will get answers from people with a bias. Some people will say Native Americans found America. Some people will say there is evidence that Africans found America. Some people say America was found when Europeans settled it. Some people will say all of the Earth's continents were connected at one time. If that's the case then those people probably walked around the land now known as America. Honestly, there is no way to know who the first person was that stepped foot on the land called America. Instead, you have to choose a qualified answer to meet your criteria or bias.