Who started the First Crusade?
If we are talking about who called the First Crusade, rather than who went on it, the only possible answer is Pope Urban II.
There were two major motives that influenced the pope in his call for this Crusade. First of all, he wanted to reunite the Eastern Orthodox Church with the Roman Catholic Church. By 1095, when Pope Urban called for the Crusade, the church in Byzantium and the church in the West had been growing apart for centuries. In 1054, this split had become much more formalized. Pope Urban wanted to reunite the church and he, secondly, wanted to reunite it under his rule.
There were also two triggering factors that led him to call the Crusade. First, the Turks were putting the Byzantine Empire in danger. The Turks, who were Muslims, were on the verge of conquering the empire and destroying Christianity in the East. Secondly, there had been an upsurge in pilgrimages to the Holy Land as Europe’s economy had revived and more people had the ability to go on pilgrimage. These pilgrims brought back stories of mistreatment by the Muslims that made the pope feel that it was necessary to conquer the Holy Land for the sake of people going on pilgrimages.