The Americas have been discovered so many times, and by so many different cultures, that no single country could be said with any confidence to have sent the first explorers. David Keys, writing for the Independent, suggests the first explorers might have predated the Asiatic migration of 15,000 years ago.
Wikipedia lists no fewer than 14 separate claims to contact with pre-Colombian America. The earliest claim to be unambiguously validated by archaeology belongs to the Norse. The first Norse explorer to see the coast of North America might have been the Norwegian Bjarni Herjólfsson, who sighted land in the summer of 986.