Are you actually 15 years old? Check with your local Game&Fish and they will steer you in the right direction. Most popular shell for duck hunting is steel shot lol, but the best way to go so your not wasting money is to pattern your gun. Start with 3 inch #3 shot. Use different chokes to find out the best shot to choke combo and you'll be in good shape.
THE RIGHT OF THE PEOPLE TO KEEP AND BEAR ARMS SHALL NOT BE INFRINGED
YOU GOT A PERMIT TO ASK STUPID QUESTIONS LIKE THAT (TRACE ADKINS)