One theory suggests that during the Ice Age (where everything on Earth was frozen–even the oceans) was the time when some of the hunter gatherer or nomadic tribes from Serbia (now in Russia) crossed the Bering Strait (A strait between the eastern end of Russia and the Western part of Alaska, today)

{Strait:- A narrow passage of water connecting two seas.} (https://en.m.wikipedia.org/wiki/File:Relief_map_of_USA_Alaska.png).

They crossed this strait unknowingly in search of food, as this is what the tribes whould do every now and then in order to survive at that time. They would relocate time to time setting camp from one place to another hunting and gathering food for their survival, hence nomadic (as farming was not known at that time hence the main source of food for the humans were other animals–more specifically meat. Even if farming was known at that time, it would have been really difficult to grow anything given the situation).

Then when the time passed, eventually the Ice Age ended and the oceans were back to being watery/liquid as they are. The people who already crossed didn't have any way back nor did they remember any way back, plus even the boats/ships were not invented at that time to cross huge bodies of water–provided if a question arises in your mind that "why didn't they use boats".

So these people eventually ended up settled in the Americas and became native to it.

This is as simple as I could explain. But if you want deep information regarding this then you can search it on YouTube, where you can easily learn more about it with visuals.