Who was the first King of the Franks who was crowned by the Pope?
The name you are looking for here is Charlemagne. This was not his actual name. His actual name was Charles. The name Charlemagne is one that was given to him much later. It means "Charles the Great."
Charlemagne was not the first King of the Franks. That title is usually given to Clovis, who lived in the late 400s and early 500s AD. However, Charlemagne is the king who was baptized by the pope and who created what is now know as the Carolingian dynasty.
Charlemagne was crowned Emperor by Pope Leo III in 800 AD. The point of this was to A) give Charlemagne and his kingdom more prestige and B) to give the Church a tie to a powerful secular ruler who could help to protect it. This was, in some ways, an attempt to recreate the situation in the latter stages of the Roman Empire in which the Church and the secular leadership reinforced one another.