The best answer for this is to say that Martin Luther was the person who started the Reformation. Luther did not start the Reformation by himself. There were other people, like John Calvin, who were important in it, but Luther is the person most usually seen as the initiator of the movement.
Martin Luther was the man who really got the Reformation started. Other people had had ideas about reforming the Catholic Church, but it was Luther who took the step that actually led to the Reformation. He did this in 1517 when he put forth a series of statements (his "95 Theses") about what was wrong with the Church. From there, he kept pursuing the idea that the Church needed to change. This was the start of the Reformation.