How can I make the mouse recognize when I click a character's body part?

The Mouse object has a Property called Target. This resembles the object your curser is on.

Get the Mouse's Target in a Button1Down event - you can use an if statement to check if the Target's Parent has a Humanoid object, and decrease health.

######(The mouse object can only be accessed from LocalScripts - put a LocalScript in any bin that replicates to the client; e.g. StarterPack, StarterGui, StarterPlayerScripts)

local plr = game.Players.LocalPlayer; --Player
local mouse = plr:GetMouse(); --Player's mouse

function LeftClick()
	local targ = mouse.Target; --This is what the mouse is on
	if targ then --If the mouse is actually on something
		--Search for a Humanoid
		local potentialHum = targ.Parent:FindFirstChild("Humanoid");
		if potentialHum then --If there's a Humanoid
			
			print("Damage da enemy >:3");

		end
	end
end

mouse.Button1Down:Connect(LeftClick) --Button1Down event on mouse