How can I do something to do something when a player joins?

Simple. You use the PlayerAdded event. But fist, we must make a function that creates the force field.

function CreateFF(player)
    Instance.new("ForceField",player.Character) --Creates a new force field in the player's character
end

Now that we've done that, all we need to do is connect() the function to the PlayerAdded event.

function CreateFF(player)
    Instance.new("ForceField",player.Character) 
end

game.Players.PlayerAdded:connect(CreateFF)

Hope I helped!

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