The Gospel of Mark is terse and has a way of getting to the point quickly. It begins with John, baptizing in the Jordan. His message echoes the Old Testament prophets. Jesus appears quickly in this story. He is a man, and comes out of Nazareth to be baptized by John.
Baptism is a pivotal event.
It is the beginning of his journey as an adult. The other Gospels also place the beginning of Jesus ministry with this event. It is not merely the beginning of the gospel, it is the beginning of Jesus calling or vocation. Nazareth and began his spiritual journey as the Messiah. He left his childhood, and accepted the responsibilities of his calling.