There’s no really definite evidence either way, yet. Many archaeologists and historians think that Native American people came from East Asia. They say that mainly because there is archaeological evidence that some new people arrived in North America about 12,000 BC. And that’s the same time that it would have been possible to walk to North America from East Asia on a land bridge. Also, recent genetic DNA evidence seems to show that North Americans are genetically related to East Asians.
Other archaeologists, and some Native people, think that Native American people originated in North America, or came to North America much earlier. They say that because the Native American traditions themselves say that. But also there is more and more archaeological and genetic evidence that at least some people lived in North America before the land bridge existed. These people may also have come over from East Asia, maybe on boats or rafts. They might have come as early as 30,000 BC. The whole question needs more work. And many historians, geneticists, and archaeologists are still working on it.
Bread Comes to Life: A Garden of Wheat and a Loaf to Eat, by George Levenson (2004). From wheat to bread, lavishly illustrated, for kids.
Ancient Agriculture: From Foraging to Farming, by Michael and Mary Woods (2000). For middle schoolers, with plenty of information about how farming got started, and how it worked.