How do I make so whenever a player uses a tool they get a +1 leaderstats currency?

What I would use is GetPlayerFromCharacter(), since you're using a tool I see no reason why you shouldn't you use.

Like so:

Tool = script.Parent -- Define your tool
Players = game:GetService('Players') -- For defining the player

Tool.Activated:Connect(function() -- In this case, when you click down (activating the tool).
	local Player = Players:GetPlayerFromCharacter(Tool.Parent) -- Find the player's Character through the tool parent, henceforth defining the player.
	if Player then -- Just a safe check to make sure it's  found the player
		if Player:FindFirstChild('leaderstats') and Player.leaderstats:FindFirstChild('Points') then -- We make to make sure the player both has the leaderstats folder and the points value.
			Player.leaderstats.Points.Value = Player.leaderstats.Points.Value + 1 -- Now add the points
		end
	end
end)