Attitude

I am new to coaching and responsible for a U15 team and experiencing a bit of trouble. Players are arriving to practice late, talking when I am giving instructions, not really working hard enough and slow to huddle up (both in practice and games) In spite of this we are generally meshing well and have won all of our games this season. I am looking for advice because I am scared of being too harsh and driving them away from the team/sport. Thank you for your help.