As they ministered to the Lord and fasted, the Holy Spirit said, "Now separate to Me Barnabas and Saul for the work to which I have called them."
Acts 13:1-2 (NKJV)
When I was involved in running sports in high school, normally, before the race began, we knew about the course we were to run and we did everything that we could to prepare ourselves. But even after we got ready and lined up at the starting point, we still had to wait for the race to officially begin. Usually there was some official who would signal the beginning of the race, but until they let us know it was time to start, we were not allowed to begin the race, and if we did, it was called a false start and we had to line up again.
In Acts 13, we see the beginning of the ministry of Barnabas and Paul, who was then known as Saul. In Acts 13:2, the Holy Spirit said, "Now separate to Me Barnabas and Saul for the work to which I have called them." We can learn several things from this. We see that there was something that Barnabas and Paul were already called to do, by God, but it was not until this point that the Holy Spirit gave them the go ahead to be separated to the work they were called to.
Barnabas and Paul were already called to do something, but it was not until the Holy Spirit gave them the signal to start that they began. Sometimes with what God has called us to do, it is a case of, ready, set, ... wait. Spending time with God, through His Word and in prayer, we learn what to do, when to do it, and how it needs to be done, we don't just decide for ourselves. Even if we know what to do, without God's timing and the leading of the Holy Spirit, we will not have the success we should, and we may not even accomplish what God is calling us to do.