Well, the obvious answer is Christopher Columbus, though he never actually reached the mainland (he only reached Hispaniola). I think it was Amerigo Vespucci that was the first European to land on the mainland (he certainly gave his name to the Continent). That is, of course, unless you think the Vikings got there first (they certainly landed in Greenland, and from there it's a fairly short hop to the Canada). Not forgetting all the Native Americans, who presumably migrated from the West accross the Bering Strait (between Russia and Alaska).