To be a leader

About three months ago, I won the department election from four candidates. I think that is a big challenge for me because I never had met any similar situations.. After that, I always asked myself how to be a good leader. Beacuse the leader of department was not like a monitor in class. A monitor just needs to care about 20~30 people but the leader of department need to worry about all students in department. So I don't know how to do it well. But it just likes the teacher said "do your best".