They walked across from Asia, back when the land masses were a lot more connected
People from asia migrated over a land bridge, connecting asia with north america. This land bridge came about when the water froze. When that happened, the water level dropped, revealing a path of land hidden under the water. The asians then followed their food (they were nomadic) across the land bridge into america. Across the thousands of years, the migration spread farther south, and eventually into south america.
The first people to live in North America migrated from Asia