How do I clone a gui into every ones startergui?

You could just place it in the StarterGui at the beginning of the game, but if you want to throw a GUI on all players - that's possible. I'm assuming that you want this GUI to come up upon a specific event, therefore I'll use clicking a button as an example.

You can fire an event to all the clients in-game using the RemoteEvent:FireAllClients() function. Let's say you got a button with a ServerScript inside of a ClickDetector. The script would look like what I have below.

script.Parent.mouseClick:connect(function(plr)
	game.ReplicatedStorage.RemoteEvent:FireAllClients(plr.Name)
end)

The local scripts would have this.

game.ReplicatedStorage.RemoteEvent.OnServerEvent:connect(function(name)
	local GUI = game.Players.LocalPlayer.PlayerGui.ScreenGui:Clone()
	GUI.TextLabel.Text = name.." pressed the button!"
	GUI.Enabled = true
end)

If this helped you out, consider accepting this answer for those sweet, sweet reputation points. If not, comment below and I (or someone else) will help you out.

Be sure to check out the Roblox API Documentation as well for additional reference.