European settlement in the New World had a number of impacts on Native Americans. Mainly, though, settlement pushed Native Americans off their lands. It led to them either dying or being forced onto reservations.
When the Europeans first came to the New World, they had two main effects. First, they brought germs that spread through Indian populations, killing huge numbers of them. The germs even killed Indians who had never seen whites because they passed from Indians who had been in contact with whites to others. Second, they brought new technologies and they brought horses. These things changed the Native American ways of life tremendously. For example, we think of the Plains Indians as horse people, but they of course had no horses until Europeans came to the Americas.
As European settlement expanded, Indians were pushed farther and farther west. Some fought wars to try to prevent this and were killed. Those who were not killed were eventually put on reservations, typically on land the Americans did not want.
In these ways, European settlement had a tremendous, and often devastating, effect on Native Americans.