Acts 13:2-4 (KJV)
2 As they ministered to the Lord, and fasted, the Holy Ghost said, Separate me Barnabas and Saul for the work whereunto I have called them.
3 And when they had fasted and prayed, and laid their hands on them, they sent them away.
4 So they, being sent forth by the Holy Ghost, departed unto Seleucia; and from thence they sailed to Cyprus.

Ministered here, according to Strong, refers to:
to be a public servant, that is, (by analogy) to perform religious or charitable functions (worship, obey, relieve): – minister. Therefore it is service to the LORD which can include: instructions in the Word of God, prayer, giving to those in need, and singing, etc. Whatever we do to serve GOD is an act of worship.

Looking at the context I believe here it is especially worship in singing and playing instruments to the LORD in adoration and praise, praying and fasting to seek direction
By worshiping, praying and fasting, as we gather together, to seek the LORD positions ourself for GOD to speak. This is also seen in the Old Testament for example in 2 Kings 3:15-16 (KJV) “But now bring me a minstrel. And it came to pass, when the minstrel played, that the hand of the LORD came upon him. [16] And he said, Thus saith the LORD, Make this valley full of ditches.”

Therefore let us be encouraged today when we need direction from the LORD in doing His great commission to gather together to worship, fast, pray, read His Word and be attentive to what the Holy Spirit is saying. As we read on in this and following chapters we see that being obedient when being sent by the Holy Spirit to mission will result in people turning to the LORD, opposition, persecution, sings, wonders, ongoing guidance, provision, protection, etc..
“Mark 16:14-20 (KJV) Afterward he appeared unto the eleven as they sat at meat, and upbraided them with their unbelief and hardness of heart, because they believed not them which had seen him after he was risen. [15] And he said unto them, Go ye into all the world, and preach the gospel to every creature. [16] He that believeth and is baptized shall be saved; but he that believeth not shall be damned. [17] And these signs shall follow them that believe; In my name shall they cast out devils; they shall speak with new tongues; [18] They shall take up serpents; and if they drink any deadly thing, it shall not hurt them; they shall lay hands on the sick, and they shall recover. [19] So then after the Lord had spoken unto them, he was received up into heaven, and sat on the right hand of God. [20] And they went forth, and preached every where, the Lord working with them, and confirming the word with signs following. Amen.”