How can i put the starter character arms togheter like a straight jacket?

use the animation editor to make a looping 1 keyframe "animation" of the pose and use a script to play the animation on CharacterAdded. if you want more functionality beyond just the pose then you'll need to script it too.

ex: (serverside script in starterplayerscripts)

local player = script.Parent
local straitjacketPose = player.Character.Humanoid:LoadAnimation(game.ServerStorage.straitjacketPose)

player.CharacterAdded:Connect(function()
    straitjacketPose:Play()
end)