North American History
The history of North America up to 1500 AD is a story of a continent that started out empty and gradually got more and more crowded. Probably the first people to come to North America came from East Asia, over a land bridge between Siberia and Alaska. We don't know for sure whether they came during an earlier Ice Age, about 30,000 years ago, or about 12,000 BC, during the last Ice Age. It could be both. Only a few people came over the bridge, with their dogs. They were probably following woolly mammoths that they were hunting for food. The people probably didn't even realize they had crossed over to North America. Probably other people came from East Asia in small boats, crossing the Pacific Ocean by following the coast around by Alaska. We don't know when that happened. There's no archaeological evidence for either method, walking or in boats, but people did get here somehow from East Asia, because we have archaeological traces of people that scientists can date to about 12,000 BC.
(This is what most archaeologists think. Some of the people whose ancestors lived in North America, like the Navajo and the Sioux, tell a different story about their own origins, which you can see here).