The first of the 12 apostles to follow Jesus were the two mentioned in John 1:35-40.
The next day John was there again with two of his disciples. 36 When he saw Jesus passing by, he said, “Look, the Lamb of God!” 37 When the two disciples heard him say this, they followed Jesus. 38 Turning around, Jesus saw them following and asked, “What do you want?”
They said, “Rabbi” (which means “Teacher”), “where are you staying?” 39 “Come,” he replied, “and you will see.”
So they went and saw where he was staying, and they spent that day with him. It was about four in the afternoon. 40 Andrew, Simon Peter’s brother, was one of the two who heard what John had said and who had followed Jesus.
So Andrew was one of the first two. The other is unnamed but is usually thought to be John, the author of the gospel.
This event took place about six months before Andrew and John were called to follow Jesus (Mark 1:16-20). The selection of the 12 apostles took place about a year later (Mark 3:13-19).