The scripture readings for today are all connected in some way with the Baptism of the Lord, which we traditionally celebrate the Sunday after the Epiphany.
The Baptism of Jesus Christ was the first major theological event in the life of the adult Jesus. Up to this point and time of His life, Jesus Had "Set aside His divinity" and lived as any ordinary human being in order to experience all the life-building joys and trials that it takes to grow from infancy to mature adult. Now, with that part of His mission completed, he is ready to take up His role in the Father's Plan of Salvation This is why the evangelists Mark and John began their Gospels at the Baptism of Christ.
John the Baptist, a relative of Jesus, had been preaching and baptizing a form of repentance that was not new to the Jews of that culture, but his was more energized and with a sincerity and urgency that the others didn't have. John's baptism was a ritual form of personal repentance and acknowledgement of one's sins.
So, you might ask, why did Jesus need to be baptized? 'The answer is more difficult to verbalize than one would think, since Jesus was "without sin." Think of it this way: Even the greatest of saints went to confession frequently and acknowledged their humanity, which is naturally prone to sin. This does not mean that Jesus or the great saints were sinners; but that they admitted to themselves and to the world that they were human and, except for the abundance of God's grace working within them, were capable of sinning.
The other theological reason for Jesus' baptism in the Jordan River by John was for those who are subsequently baptized in the Name of Jesus after His death and resurrection, what was once just a personal pious act now becomes a commitment to God through Jesus Christ. Also, the Holy Spirit is released into the soul of the penitent, thereby overcoming the consequences of original sin. This is why Jesus elevated this ritual to a sacrament.