How would you change a player's ShirtTemplate when the enter a game?

Copy and paste id from shirts and pants + there is no need to subtract 1 from them

Why yours was wrong:

-Data hasnt loaded by the time :findFirstChild executes so your left with nil -The Parent of player is Players which isnt a character so you need to wait for chr to load by using

Example of how to fix this issue:

Player.CharacterAdded:connect(function(chr)
	--stuff
end)

Here is your script:

local ShirtId = 153367418
local PantsId = 110454553

game.Players.PlayerAdded:connect(function(plr)
	plr.CharacterAdded:connect(function(chr)
		while true do
			shirt = chr:findFirstChild("Shirt")
			pants = chr:findFirstChild("Pants")
			if shirt and pants then
				break
			else
				wait()
			end
		end
		shirt:Destroy()
		pants:Destroy()
		local NewShirt = Instance.new("Shirt",chr)
		local NewPants = Instance.new("Pants",chr)
		NewShirt.ShirtTemplate = "http://www.roblox.com/asset/?id="..ShirtId-1
		NewPants.PantsTemplate = "http://www.roblox.com/asset/?id="..PantsId-1
		print("Changed "..plr.Name.."'s attire.")
	end)
end)