Can somebody help with this script?

i am trying to make a script that when you click on a button it makes a decal pop up on a part. I have started it but i can't think of the rest and how it will work.Thanks!

local screen = game.workspace.screen
local button = game.workspace.button

function onClick

Don't delete my answer this time man. :(

What this code does is, on click the button creates a decal and puts it on the Screen.

local screen = workspace.screen
local button = workspace.button
local ClickD = Instance.new('ClickDetector', button)

ClickD.MouseClick:connect(function()
local Decal = Instance.new('Decal', screen)
end)