Matthew 20:22 James, John, and their mother failed to grasp Jesus' previous teaching on rewards (19:16-30) and eternal life (20:1-6). They failed to understand the suffering they must face before living in the glory of God's kingdom. The "cup" was the suffering and crucifixion that Christ faced. Both James and John would also face great suffering. James would be put to death for his faith, and John would be exiled.
Matthew 20:23 Jesus was showing that He was under the authority of the Father, who alone makes the decisions about leadership in heaven. Such rewards are not granted as favors. They are for those who have maintained their commitment to Jesus in spite of severe trials.
Matthew 20:24 The other disciples were upset with James and John for trying to grab the top positions. All the disciples wanted to be the greatest, but Jesus taught them that the greatest person in God's kingdom is the servant of all. Authority is given not for self-importance, ambition, or respect, but for useful service to God and His creation.
Matthew 20:27 Jesus described leadership from a new perspective. Instead of using people, we are to serve them. Jesus' mission was to serve others and to give His life away. A real leader has a servant's heart. Servant leaders appreciate others' worth and realize that they're not above any job. If you see something that needs to be done, don't wait to be asked. Take the initiative and do it like a faithful servant.