How to make a script that kills the player on join?

First your going to want to use a PlayerAdded event

game.Players.PlayerAdded:connect(function(player)
--Code Goes here
end)

Then your going to want to kill the player, the way you can do this is by waiting for the characater then getting the humanoid and setting the health to zero and removing the forcefield.

game.Players.PlayerAdded:connect(function(player) --Waits for player
    player.CharacterAdded:connect(function(char) --Waits for character
        wait() --Waits so they can kill
        char:WaitForChild("ForceField"):Destroy() --Destroys ForceField
        char:WaitForChild("Humanoid").Health=0 --Sets Health to zero
    end)
end)

That should work

I suggest this being a Server Script:

game:GetService("Players").PlayerAdded:Connect(function(plr)
    local character = plr.CharacterAdded:Wait()
    character:WaitForChild("ForceField"):Destroy()
    character:WaitForChild("Humanoid").Health = 0
end)

Hope this works!