First, Jesus called them. He brought them together so that He could teach them how to work in matters of the Spirit and of the world.
Then He gave them power and authority over the demons (the unseen, spiritual enemies) and the diseases (the elements of the world, physical and mental.) Giving them the power meant they had the ability to use this gift and giving them the authority meant they had the choice to use it with a presence that would be highly effective because the Lord would stand behind them.
Finally, Jesus sent them out. They were to proclaim the Kingdom of God, which was to share the Good News of Jesus the Messiah and the power of salvation to all they meet. They were also to perform healing -- to cleanse and renew the people physically, mentally and spiritually.
Jesus does the same for us today. He calls us. He gives us power (ability) and authority (choice) over the things of the physical and spiritual worlds. And then He sends us out to tell others about God's mercy and love. We are then to renew, help and heal the world in a physical and spiritual way.