How do you make the arm point towards where the player is looking?

So I made a script where if the user equips the tool their arm will point to where they are looking:

tool.Equipped:connect(function()
	debounce = true
	while debounce do
		
		wait()
		mouse = plr:GetMouse()
		rsh = tool.Parent:FindFirstChild("Torso")["Right Shoulder"]
		pos = Vector3.new(0,0,0)
		rsh.C1 = rsh.C1 * CFrame.new(pos,mouse.Origin.p)
	end
end)

tool.Unequipped:connect(function()
	debounce  = false
end)

But when I use the script the arm the joint is in the same position but is randomly pointing to random positions even when the cursor is still.

This is only a partial answer.

The reason the arm is spinning wildly is that you aren't returning the shoulder to its original state each frame. To undo multiplying by a CFrame, you can multiply by its inverse.

Minor: debounce is the variable name traditionally used to prevent a function from running multiple times simultaneously. You're using it as a "continue to operate" variable (maybe just call it "equipped", "keepRunning/Executing", or just "run/execute"?)

Here's a partially fixed script:

local tool = script.Parent
local localPlayer = game.Players.LocalPlayer
local lastcf, rsh, run
tool.Equipped:connect(function(mouse)
	run = true
	lastcf = nil
	while run do
		wait(0.1)
		rsh = localPlayer.Character.Torso["Right Shoulder"]
		if lastcf then
			rsh.C1 = rsh.C1 * lastcf:inverse()
		end
		local cf = CFrame.new(Vector3.new(0, 0, 0), mouse.Origin.lookVector) * rsh.C1:inverse()
		rsh.C1 = rsh.C1 * cf
		lastcf = cf 
	end
	if lastcf then
		rsh.C1 = rsh.C1 * lastcf:inverse()
	end
end)

tool.Unequipped:connect(function()
	run = false
end)

Also, note that using the character's Torso won't work with the R15 character models.

Further, rsh = game.Players.LocalPlayer.Character.Torso["Right Shoulder"] is error prone -- if the Torso doesn't exist, the script will error. Using FindFirstChild("Torso") doesn't help if you then don't check if it returns nil! We can rewrite that bit like so:

		local torso = localPlayer.Character:FindFirstChild("Torso")
		if not torso then return end
		rsh = torso["Right Shoulder"]

The problem with the script is the line local cf = CFrame.new(Vector3.new(0, 0, 0), mouse.Origin.lookVector) * rsh.C1:inverse(). I know that you want the lookVector of mouse.Origin (the direction that the user is looking), not mouse.Origin.p (which is sort of the position of the camera), but I don't know what to do with it after that.