See the notes at Matthew 3:2.

Jesus began to preach, and to say, Repent - See on Matthew 3:1, Matthew 3:2 (note). Every preacher commissioned by God to proclaim salvation to a lost world, begins his work with preaching the doctrine of repentance. This was the case with all the prophets, John the Baptist, Jesus Christ, all the apostles, and all their genuine successors in the Christian ministry. The reasons are evident in the notes already referred to; and for the explanation of the word κηρυσσειν, preaching or proclaiming as a herald, see at the end of chap. 3 (note).

From that time Jesus began to preach, and to say, Repent: for the kingdom of heaven is at (g) hand.
(g) Is come to you.

From that time Jesus began to preach and to say,.... Not from the time he dwelt in Capernaum; for he had preached in Nazareth before he came there, Luke 4:16 nor from the time of John's being cast into prison; for he had preached, and made disciples, who were baptized by his orders, before John's imprisonment, John 3:22 John 4:1 but from the time that Satan left tempting him; as soon as that combat was over, immediately he went into Galilee, began to preach, and called his disciples. The words with which he began his ministry are the same with which John begun his; which shows the entire agreement between them, in that they not only preached the same doctrine, but in the same words; See Gill on Matthew 3:2

From that time Jesus began to preach, and to say, Repent; for the kingdom of heaven is at hand--Thus did our Lord not only take up the strain, but give forth the identical summons of His honored forerunner. Our Lord sometimes speaks of the new kingdom as already come--in His own Person and ministry; but the economy of it was only "at hand" until the blood of the cross was shed, and the Spirit on the day of Pentecost opened the fountain for sin and for uncleanness to the world at large.
Calling of Peter and Andrew James and John (Matthew 4:18-22).

From that time. Probably from the time of the settlement of Jesus in Capernaum.
Jesus began to preach. This is the beginning of the Galilean ministry.
And to say, Repent, etc. The message that Jesus now preaches is identical with that of John the Baptist. See Matthew 3:2. He commands repentance, and declares the kingdom of heaven is at hand, not yet come, but near. All is still preparatory. Jesus had not yet declared himself as the Messiah.

From that time Jesus began to preach - He had preached before, both to Jews and Samaritans, John 4:41, John 4:45. But from this time begin his solemn stated preaching. Repent, for the kingdom of heaven is at hand - Although it is the peculiar business of Christ to establish the kingdom of heaven in the hearts of men, yet it is observable, he begins his preaching in the same words with John the Baptist: because the repentance which John taught still was, and ever will be, the necessary preparation for that inward kingdom. But that phrase is not only used with regard to individuals in whom it is to be established, but also with regard to the Christian Church, the whole body of believers. In the former sense it is opposed to repentance; in the latter the Mosaic dispensation.