I need to use DataSaving / Persistence, but how?

Ok, let me tell you everything. I am planning to make a computer system for my place. Say YOU join as a new player and try to log onto the computer but you need to sign up, how do I save the Sign Up data that YOU have inputted then say you leave and comeback and try to log on with that data. I know you have to use Data Persistence or something like that. Here is some test code trying THIS idea out not the computer yet.

local a = Instance.new("StringValue")

game.Players.PlayerAdded:connect(function(player)
    if player:WaitForDataReady() then
 		local b = player:LoadInstance(player.Name.."'s password")
		b.Parent = game.ServerStorage.Passwords
		player.PlayerGui.ScreenGui.TextBox.Text = game.ServerStorage:FindFirstChild(player.Name.."'s password").Value
	 end   
end)

game.Players.PlayerRemoving:connect(function(player)
    if player:WaitForDataReady() then
	if not game.ServerStorage.Passwords:FindFirstChild(player.Name.."'s password")then
		a.Value = player.PlayerGui.ScreenGui.TextBox.Text
		a.Parent = game.ServerStorage.Passwords
		player:SaveInstance(player.Name.."s' Password",a)
	   end 
	end
end)