Making a player deal damage with a click?

I'm trying to make a simple fps game, I already have the main stuff done for the gun system. I just need to make it do damage, So i'm thinking when you click a player it would deal maybe 15 damage, but clicking the player's head would deal 25. I'm mostly a builder so this stuff isn't really what I'm best at, but I'm learning if anybody knows how to do this could they please tell me.