Acts 9:26 And when Saul was come to Jerusalem, he assayed to join himself to the disciples: but they were all afraid of him, and believed not that he was a disciple. 9:27 But Barnabas took him, and brought him to the apostles, and declared unto them how he had seen the Lord in the way, and that he had spoken to him, and how he had preached boldly at Damascus in the name of Jesus. 9:28 And he was with them coming in and going out at Jerusalem. 9:29 And he spake boldly in the name of the Lord Jesus, and disputed against the Grecians: but they went about to slay him. 9:30 Which when the brethren knew, they brought him down to Caesarea, and sent him forth to Tarsus. 9:31 Then had the churches rest throughout all Judaea and Galilee and Samaria, and were edified; and walking in the fear of the Lord, and in the comfort of the Holy Ghost, were multiplied.
Where was Tarsus?
In Cilicia.
Acts 21:39 But Paul said, I am a man which am a Jew of Tarsus, a city in Cilicia, a citizen of no mean city: and, I beseech thee, suffer me to speak unto the people.
What did he do there?
A bit more on what Acts 9 related...
Galatians 1:11 But I certify you, brethren, that the gospel which was preached of me is not after man. 1:12 For I neither received it of man, neither was I taught it, but by the revelation of Jesus Christ. 1:13 For ye have heard of my conversation in time past in the Jews' religion, how that beyond measure I persecuted the church of God, and wasted it: 1:14 And profited in the Jews' religion above many my equals in mine own nation, being more exceedingly zealous of the traditions of my fathers. 1:15 But when it pleased God, who separated me from my mother's womb, and called me by his grace, 1:16 To reveal his Son in me, that I might preach him among the heathen; immediately I conferred not with flesh and blood: 1:17 Neither went I up to Jerusalem to them which were apostles before me; but I went into Arabia, and returned again unto Damascus. 1:18 Then after three years I went up to Jerusalem to see Peter, and abode with him fifteen days. 1:19 But other of the apostles saw I none, save James the Lord's brother. 1:20 Now the things which I write unto you, behold, before God, I lie not. 1:21 Afterwards I came into the regions of Syria and Cilicia; 1:22 And was unknown by face unto the churches of Judaea which were in Christ: 1:23 But they had heard only, That he which persecuted us in times past now preacheth the faith which once he destroyed. 1:24 And they glorified God in me.
So, he went off to Syria and Cilicia (which is where Tarsus is).
What did he do there?
Acts 11:25 Then departed Barnabas to Tarsus, for to seek Saul: 11:26 And when he had found him, he brought him unto Antioch. And it came to pass, that a whole year they assembled themselves with the church, and taught much people. And the disciples were called Christians first in Antioch.
So Barnabas joins Paul in Tarsus, and returns with him to Antioch.
Again, though - what did Paul do while in Syria and Cilicia (which is where Tarsus is)?
Acts 15:22 Then pleased it the apostles and elders, with the whole church, to send chosen men of their own company to Antioch with Paul and Barnabas; namely, Judas surnamed Barsabas, and Silas, chief men among the brethren: 15:23 And they wrote letters by them after this manner; The apostles and elders and brethren send greeting unto the brethren which are of the Gentiles in Antioch and Syria and Cilicia: 15:24 Forasmuch as we have heard, that certain which went out from us have troubled you with words, subverting your souls, saying, Ye must be circumcised, and keep the law: to whom we gave no such commandment:
15:30 So when they were dismissed, they came to Antioch: and when they had gathered the multitude together, they delivered the epistle: 15:31 Which when they had read, they rejoiced for the consolation.
15:36 And some days after Paul said unto Barnabas, Let us go again and visit our brethren in every city where we have preached the word of the LORD, and see how they do.
15:40 And Paul chose Silas, and departed, being recommended by the brethren unto the grace of God. 15:41 And he went through Syria and Cilicia, confirming the churches.
Galatians 2:1 Then fourteen years after I went up again to Jerusalem with Barnabas, and took Titus with me also. 2:2 And I went up by revelation, and communicated unto them that gospel which I preach among the Gentiles, but privately to them which were of reputation, lest by any means I should run, or had run, in vain.
Acts 21:18 And the day following Paul went in with us unto James; and all the elders were present. 21:19 And when he had saluted them, he declared particularly what things God had wrought among the Gentiles by his ministry. 21:20 And when they heard it, they glorified the Lord, and said unto him, Thou seest, brother, how many thousands of Jews there are which believe; and they are all zealous of the law: 21:21 And they are informed of thee, that thou teachest all the Jews which are among the Gentiles to forsake Moses, saying that they ought not to circumcise their children, neither to walk after the customs.
21:25 As touching the Gentiles which believe, we have written and concluded that they observe no such thing, save only that they keep themselves from things offered to idols, and from blood, and from strangled, and from fornication.
Acts 17:11,12
Rom. 5:8.