How do I get a working light that can be turned off and on?

You can make a light var to check if it is on or off by using if statements! We check if the light var is false to turn it on and make it true. The else keyword will have a code inside when the var is true to be false and turn off the light.

local light = false
script.Parent.ClickDetector.MouseClick:Connect(function()
    if light == false then
            script.Parent.Transparency = 0
            script.Parent.Material = "Neon"
            script.Parent.BrickColor = BrickColor.new("New Yeller")
            script.Parent.PointLight.Enabled = true
        light = true
    else
            script.Parent.Transparency = 0.5
         script.Parent.Material = "Plastic"
            script.Parent.BrickColor = BrickColor.new("Dark stone grey")
            script.Parent.PointLight.Enabled = false 
        light = false
end
end)

Explanation:

The variable 'light' is currently set to false. Once the event fires, it checks if the light variable is true or false.

If true then, it will do:

script.Parent.Transparency = 0 -- Makes the transparency 0
script.Parent.Material = "Neon" -- Change Material to 'Neon'
script.Parent.BrickColor = BrickColor.new("New Yeller") -- 'Brick color 'New Yeller''
script.Parent.PointLight.Enabled = true -- PointLight property = true
light = true  -- The var will turn to true in order to turn it off when clicked again since the if statement checks if it is false or true

If false:

script.Parent.Transparency = 0.5 -- Makes the transparency 0.5
script.Parent.Material = "Plastic"-- Change Material to 'Plastic'
script.Parent.BrickColor = BrickColor.new("Dark stone grey")-- Brick color 'Dark stone grey'
script.Parent.PointLight.Enabled = false  -- PointLight property = false
light = false -- The var will turn to false in order to turn it on when clicked again since the if statement checks if it is false or true