Theme Number 31 Barnabas and Saul called and sent off on the first Missionary Journey.

Acts 13: 1-3
Act 13:1  Now there were in Antioch, in the Church there--as Prophets and teachers--barnabas, Symeon surnamed 'the black,' Lucius the Cyrenaean, Manaen (who was Herod the Tetrarch's foster-brother), and Saul.
Act 13:2  While they were worshipping the Lord and fasting, the Holy Spirit said, "Set apart for Me, now at once, Barnabas and Saul, for the work to which I have called them."
Act 13:3  So, after fasting and prayer and the laying on of hands, they let them go.
Act 13:4  They therefore, being thus sent out by the Holy Spirit, went down to Seleuceia, and from there sailed to Cyprus.WNT

The call of God to the Work. v.2.
Act 13:2  While they were worshipping the Lord  [ministering to the Lord] and fasting, the Holy Spirit said, "Set apart for Me, now at once, Barnabas and Saul, for the work to which I have called them."

[ministering to the Lord] λειτουργέω leitourgeō
Thayer Definition:
2) to do a service, perform a work 2a) of priests and Levites who were busied with the sacred rites in the tabernacle or the temple
2b) of Christians serving Christ, whether by prayer, or by instructing others concerning the way of salvation, or in some other way 2c) of those who aid others with their resources, and relieve their poverty
Vincent’s Word Studies Ministered (λειτουργούντων)
See on the kindred noun ministration, Luk_1:23. This noun has passed through the following meanings: 1. A civil service, especially in the technical language of Athenian law. 2. A function or office of any kind, as of the bodily organs. 3. Sacerdotal ministration, both among the Jews and the heathen (see Heb_8:6; Heb_9:21). 4. The eucharistic services. 5. Set forms of divine worship (Lightfoot, “On Philippians,” ii., 17). Here, of the performance of Christian worship. Our word liturgy is derived from it.
and fasting Many devout Jews fasted Mondays and Thursdays. This may have been one of those days.
“Very likely that this was a fast appointed and fixed among themselves, on some particular occasion; it may be on account of the famine, which was at this time, Act_11:28. “ J.G.
They also were facing the Responsibility of Taking the Good News to the world. There is no better time to fast as when you are seeking Heaven’s direction.
The Holy Spirit said, "Set apart for Me, now at once, Barnabas and Saul, for the work to which I have called them."

The Holy Spirit said,”Set apart for Me, [ Separate unto Me]....”
The Holy Ghost said; either with an articulate voice, or by an internal impulse, upon the minds of three of the prophets:
Separate :The Greek adds δή, now, which is not rendered by A. V. or Rev. It gives precision and emphasis to the command, implying that it is for a special purpose, and to be obeyed at the time. Compare Luk_2:15; Act_15:36; 1Co_6:20.
Set apart phorizō Thayer Definition: 1) to mark off from others by boundaries, to limit, to separate
1a) in a bad sense: to exclude as disreputable 1b) in a good sense: to appoint, set apart for some purpose.
for the work to which I have called them." the work which the Holy Ghost had appointed, and called them to before this, was to go and preach the Gospel among the Gentiles, distinct from the Jews, of which we read in the latter part of this chapter; and now he would have them be separated from their brethren, as Aaron and his sons were from theirs, and be sent forth from hence directly, upon that work: this shows the Spirit to be a person, since speaking and commanding in an authoritative way, and calling to a work, are ascribed unto him.” J.Gill
Note: The Direction of the Holy Spirit was a confirmation of what they had been previously called to. They now were to begin this ministry.