How do i make when players join they get ff forever?

Place this in a normal script in the workspace.

game.Players.PlayerAdded:connect(function(p)
    p.CharacterAdded:connect(function(c)
        Instance.new("ForceField", c)
    end)
end)

Explanation:

When the player joins the game, it waits for the players character to be added. Once it is added it adds a ForceField object to the player's character.

You can also set the Duration property of a SpawnLocation to a number obscurely high. Scrptury's way is suggested, though.