How would you make a stat system and level system?

I've done this with leaderstats before and its easy that way.

I want to know how to do it without so its harder to exploit/change with exploits.

You should keep all the real values somewhere that the client cannot access them. I personally use ServerStorage. Then you can have a Changed event linked to those values and make the leaderboard display them. Here is an example:

game.Players.PlayerAdded:Connect(function(player)

	local stats = Instance.new("Folder")
	stats.Name = "leaderstats"
	stats.Parent = player

	local cash = Instance.new("IntValue")
	cash.Name = "Cash"
	cash.Parent = stats

	local playerFolder = Instance.new("Folder")
	playerFolder.Name = player.Name -- every time you need to access the players real money from a script you will need to find the folder with the players name in ServerStorage
	playerFolder.Parent = ServerStorage

	local serverCash = Instance.new("IntValue")
	serverCash.Name = "ServerCash"
	serverCash.Parent = playerFolder

	serverCash.Changed:Connect(function()
		-- whenever serverCash gets changed by you the leader board will now display that amount without it being vulnerable to attack
		cash.Value = serverCash.Value

	end)
end)

Note: You will have to remember to :Destroy() the folder when the player leaves. else server storage could quickly be filled up with folders. Hope this helps and have a great day scripting!

I remember seeing something where you would have another two values and if the real values were different to the other two then it would change back. So basically say when a player earns 1 cash it updates the two different values and then the real values contain a script were if it notices a change it will update the real values.

Hope this helps.