How do I add a ForceField to a character every time it spawns?

You simply create a new ForceField object in the character model.

This should do the trick:

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

Just apply a ForceField to the character model and it should do! So just have a script that detects when the character respawns and then add a forcefield... :)

Game:GetService"Players".PlayerAdded:connect(function(Player) 
    Player.CharacterAdded:connect(function(Character)
        Instance.new("ForceField",Character)
    end)
end)

wow 9 seconds.... really....

Do you mean just every time or forever because you can just click the force field enable on the spawn properties. Much easier than a script.