Leif Erikson was an explorer from Iceland who was on a voyage where it is possible that he was the first to spot the shore of what is now known as the Americas. It is possible he wasn't the very first to spot the shore however, as there were others on the voyage. He was born almost 500 years BEFORE Christopher Columbus.
Christopher Columbus is typically referred to as the person who discovered America, however, he didn't actually know that it was a new land. He also never set foot in North America. Because he didn't know this was a new land, he technically did not "discover" America.
Amerigo Vespucci was the one who officially discovered the land Columbus landed on was a new land, now known as the Americas. The Americas were even named after him.
In schools, however, they normally teach that Columbus was the one who "discovered" America, so I would go with C. Christopher Columbus.
