How did Muhammad become a prophet and a messenger of God?

At the age of 40, while engaged in a meditative retreat, Muhammad received his first revelation from God through the Angel Gabriel. This revelation, which continued for twenty-three years, is known as the Quran.

The Mountain of Light
where Gabriel came to
Prophet Muhammad. 

As soon as he began to recite the words he heard from Gabriel, and to preach the truth which God had revealed to him, he and his small group of followers suffered bitter persecution, which grew so fierce that in the year 622 God gave them the command to emigrate. This event, the Hijra, 'migration', in which they left Makkah for the city of Madinah some 260 miles to the north, marks the beginning of the Muslim calendar.
After several years, the Prophet and his followers were able to return to Makkah, where they forgave their enemies and established Islam definitively. Before the Prophet died at the age of 63, the greater part of Arabia was Muslim, and within a century of his death Islam had spread to Spain in the West and as far East as China.

The Prophet's Mosque, Madinah, 
the dome indicates the place where 
his house stood and where he is buried.