In Acts 4:36f it is recorded that a man named Joseph, who was a Levite from Cyprus, sold a field that the owned and laid the proceeds at the apostles’ feet.  For this generous action, and for various others, he was dubbed ‘Barnabas’, meaning ‘Son of Encouragement’.  Acts 11:23 gives another example of his ministry of encouragement.  Furthermore,

1. He introduced the newly-converted Saul to the apostles and convinced them of his sincerity, Acts 9:27; cf. Gal 1:18;
2. He represented the apostles at Antioch, defending the conversion of Gentiles there as a work of the Holy Spirit and as a suitable sphere of ministry for Saul, Acts 11:19f;
3. He demonstrated full commitment to the conversion of Gentiles (cf Acts 13:46), and journeyed with Paul (Acts 13-14 as they established Gentile churches in Cyprus (his own home) and throughout Asia Minor.  (Note that up until the visit to Cyprus the pair are referred to as ‘Barnabas and Saul’; thereafter, it is ‘Paul and Barnabas’.
4. He appears to have had a prominent role when the Jerusalem Council considered the circumcision question, Acts 15:1-29.
5. Barnabas did not withstand Peter’s temporary Judaizing tendency, and ‘even he’ broke table-fellowship with the Gentile believers in Antioch, Gal 2:13.  But a certain strength of character is indicated when he insisted on including his cousin Mark (who had previously deserted them) on a proposed missionary journey.  When Paul refused, Barnabas took Mark with him the Cyprus (Acts 15:36-40).  Paul’s later favourable comments about Mark (2 Tim 4:11) suggest that Barnabas’ mentorship of Mark had been successful.
6. Like Paul (but unlike most of the apostles) Barnabas supported himself financially, 1 Cor. 9:6.

(Based on an article by A.F. Walls in NBD)