How do I put someone in first person locked?

game.Players.PlayerAdded:connect(function(plr)
    plr.CameraMode = "LockFirstPerson"
end)

It's quite simple. In the Player object, there is a property called CameraMode. Set it to LockFirstPerson or 1. I recommend putting a LocalScript in StarterGui with the following code:

game.Players.LocalPlayer.CameraMode = 1

Here's the script I use in my game:

script.FirstPersonLock:Clone().Parent = game.StarterGui
script:Destroy()

Put a LocalScript inside of that containing this:

local lPlr = game.Players.LocalPlayer
if lPlr then
    wait()
    lPlr.CameraMode = 0
    lPlr.CameraMode = 1
end
script:Destroy()