Chapter 4:18-22 The First Disciples

4:18 And Jesus, walking by the sea of Galilee, saw two brethren, Simon called Peter, and Andrew his brother, casting a net into the sea: for they were fishers.

When Christ began to preach, He began to gather disciples, who should now be the hearers, and hereafter the preachers, of His doctrine.
Q: Why did not Jesus Christ call some of the eminent Scribes or Pharisees to publish his Gospel, and not poor unlearned fishermen, without credit or authority?
A: Because it was the kingdom of heaven they were to preach, and their teaching must come from above.

4:19 And He saith unto them, Follow Me, and I will make you fishers of men.

For, I "will make you fishers of men": you shall be fishers still, but in a higher sense.

4:20 And they straightway left their nets, and followed Him.

And they straightway left their nets - That is, as soon as He had called them, they left their worldly employment, and followed Him; they gave up themselves to his service, and became His disciples; they not only left their "nets", but their fishing boats, and fishing trade, and all that belonged to it.

4:21 And going on from thence, He saw other two brethren, James the son of Zebedee, and John his brother, in a ship with Zebedee their father, mending their nets; and He called them.

Follow Me - Come after Me, δευτε οπισω μου. Receive My doctrines, imitate Me in My conduct - in every respect be My disciples. 

We may observe that most of the calls of God to man are expressed in a few solemn words, which alarm the conscience, and deeply impress the heart.

4:22 And they immediately left the ship and their father, and followed Him.

And they immediately left the ship - More is expressed here than before, for they not only left their nets, but their ship too; which was of much more value.