How can I use FindfirstChild?

I was trying to make an Backpack Gui, So when the player can see the image slot. But how to use FindfirstChild instead of using Parents?

script:

local U = game:GetService("UserInputService")
local debounce = true
U.InputBegan:Connect(function(key) --Not interested to use InputEnded so.
    if key.KeyCode == Enum.KeyCode.One then
        if debounce == true then
            debounce = false
            print("On!")
            script.Parent. -- I don't know how to use FindFirstChild yet.
        else
            debounce = true
            print("Off.")
        end
    end
end)