How do I award kill credit to the players?

So in the game that me and my friend are creating we have enemy NPC's that give gold and xp when killed. Currently, when the NPC's are killed it gives gold and xp to all players in the game, even if they haven't attacked it. What I want to happen is that either the person that deals the most damage gets the rewards, or the person that deals the last hit get the reward. There isn't any sources that I can find that give the details on who to give the rewards too...