Check out The Matrix: “Don’t think you can, know you can”. First, know what the captain needs to know. Ask the previous captain, if available, for advice. Second, nobody’s perfect. You don’t have to be perfect either, everybody learns by doing. Third, listen to advice, but make your own decisions. People respect a captain who is honest, fair, can make their own decisions, and puts the team over themselves. People who are captains are not in it for glory or popularity. They do it because they care and they believe in the best interest of the group. Know why you are doing it, and confidence will come.