How did christopher Columbus find a new route to Asia?

As a matter of fact, he didn't. Other explorers that came later finally found the way West to Asia, but no Northwest passage was discovered. Whomever found the western passage first (I forgot who; sorry) had to go past the tip of South America. As a side note: contrary to popular belief, everybody in Columbus' time with a decent education knew the Earth was round. Also, Columbus was a terrible leader. He was allowed to keep control of his colonies in the New World because he sent false reports to the queen of the riches he had found there. At one point, he was almost killed by a group of colonists when they got tires of his horrible leadership. Once Columbus returned to England following his fifth voyage over (it took him 3 to find land) he was beheaded on the spot.