ANSWER: He became king of the Franks and then conquered other lands.
STEP BY STEP EXPLANATION:
Charlemagne became king of the Franks after the death of his father, Pepin the Short.
He inherited a sizeable portion of land that included what is now France, the Low Countries, and parts of Germany.
Charlemagne launched several military campaigns throughout his long reign (he ruled for 46 years), and was highly successful in most of them. He conquered Northern and Central Italy, what is now Switzerland and Austria, and the majority of modern day Germany.
He was crowned Holy Roman Emperor in the year 800 by Pope Leo III. In exchange for this, Charlemagne ceded a section of Central Italy to the Pople: the Papal States, which would be under direct control of the Roman Catholic Church until the Napoleonic Era.