Head over to Aredia City and talk to the man standing on the blue carpet (Picture shown below) and he will give you the option of choosing either Zacian or Zamazenta. Once you choose one, you will be given a quest to kill 280 Pokémon. Once you kill 280 Pokémon, Go back and talk to the man and he will give you the Pokémon and the item to get Crowned Zacian (Rusted Sword) or Zamazenta (Rusty Shield) depending on who you picked. 

Note that you can talk to the NPC again whenever you'd like to keep track of the amount of Pokémon you've killed.
Here is a video that helps!