On Paul’s first journey, he had travelled with Barnabas (Acts 13:1-3). But when Paul wanted to set out on his second journey, he had an argument with Barnabas about whether John Mark should also come (Acts 15:36-38). In the end, Barnabas went with John Mark to Cyprus (Acts 15:39) and Paul went with Silas through Syria and Cilicia (Acts 15:40-41).
Silas was with Paul for much of his travels on the second journey, as was Timothy who joined them at Lystra. The last we hear of Silas is when he caught up with Paul in Corinth (Acts 18:5) after spending some extra time in Macedonia while Paul had gone on ahead. We don’t know if Silas travelled with Paul back to Antioch or whether he left Corinth on his own missionary travels.