Being a leader isn’t something that happens over night.  You can’t show up to practice one day and decide you’re going to be a leader.  That being said, however, there has to be a concious decision, on your part, that you intend to put the work in to be a leader.  A common misconception is that if you’re the best player on the team, you’re the captain.  Wrong! 
The leader is someone who can hold people accountable for their actions, but not allow them to fail.  A leader is someone who understands the weak/strong points of the team and works off of them.  A leader is someone who’s focus is on becoming a better team, not necessarily winning all the time — winning will come if the pieces are in place. 
Don’t get sucked into the vacuum of thinking that you need a captain’s arm-band around your arm to be considered a leader.  Make a concrete effort to make the players around you better; the coach and players will begin to notice this and view you as a leader.