Making a restart button for my game that resets all data.. how?

I am assuming here you are capable of making a GUI Button. The solution to this is straight forward, just set the DataStore data for the player to the default data when the player clicks the button. Since the GUI button would be client-side, you would need to send a remote signal to the server to do the data clearing.

For example, your scripts may look something like this:

-- local script
local guiButton = reference.to.gui.button
local remoteEvent = reference.to.remote.event

guiButton.Activated:Connect(function(inputObject)
	remoteEvent:FireServer()
end)

-- server script
local datastore = game:GetService('DataStoreService')
local store_playerData = datastore:GetDataStore('playerData')
local defaultData = {
	-- Set the default data
	Money = 0,
	Stats = 0,
	Level = 0,
}

remoteEvent.OnServerEvent:Connect(function(player)
	-- Get access to the player's datastore 
	local playerKey =  "Player_"  .. player.UserId
	local success, err =  pcall(function()
		store_playerData:SetAsync(playerKey,  defaultData)
	end)

	if not success then print(err) end
end)