It is proper for us to do this to fulfill all righteousness

Matthew 3:11-17
It is proper for us to do this to fulfill all righteousness. So John baptized Jesus.
As soon as Jesus was baptized, he went up out of the water. At that moment heaven was opened, and he saw the Spirit of God descending like a dove and alighting on him.  And a voice from heaven said, “This is my Son, whom I love; with him I am well pleased.
Why?
Jesus is the son of God, the Holy Spirit and the voice from heaven witnessed Him.
Righteousness is obey the Lord and follow his commands.
Jesus was baptized so we should be baptized either then we may serve the Lord and other people.
Jesus had no need of this, as verses 14-15 and the rest of the New testament make plain. He had no need to be baptized in order to be forgiven because he 'had no sin'. 
Yet it is right, 'for now', because Jesus must  do 'all that God requires' to save the world. So he steps into the River Jordan, accepting his destiny, to received God's commission and blessing.
Jesus is a man at that moment.
Matthew has already shown that Jesus is the Messiah.
How?
Fulfill the righteousness of the Lord, do the right things. Even lacking the water now I won't take water from my neighbor without notice.