How to make the character's head move to face the mouse?

So, this is hard to explain. I am currently working on a fps stealth game. Now, for the game I want to make the player be able to look up and down with the mouse, so if mouse is up, character looks up, mouse is down, looks down. Something along those lines. Now I did make a script for this, only problem, it's not r15 compatible. Oof. I can't find any tutorial for doing this with r15 so that's why I am here, can anybody help me out. Don't send a complete script, maybe explain it and give some functions to use, and the math? I am just confused, and I will like some help.

local Players = game:GetService("Players")
mouse = Players.LocalPlayer:GetMouse()



 target = mouse.p
 Players.LocalPlayer.Character:FindFirstChild("Head").Rotation = (haven't figured this out yet)