how would i make a tool add leaderstats?

Well, You at least took the effort to do some research.. You're going to have to make use of Variables, Math, Events, Functions, and Connections.

Usually people use local scripts for tools but if we want to make an actual change to a value in leaderstats then I recommend the use of a server script.

We're going to make a tool that add 1 to a leaderstats value then the tool disappears.

-- Server Script --

-- Variables

local tool = script.Parent -- defining tool

-- Events/Connections

tool.Activated:Connect(function() -- tool "Activated" Event
	
	local character = tool.Parent -- defining character
	local players = game:GetService("Players") -- defining players
	local player = players:GetPlayerFromCharacter(character) -- defining player
	local leaderstats = player.leaderstats -- defining leaderstats
	local currency = leaderstats.Money -- defining currency
	local addition = 1 -- defining addition

	currency.Value = currency.Value + addition -- Adding 1 to Currency
	
end)

Of course.. This script may error in certain circumstances.. But that's for the better. So you can observe, problem-solve, and understand the errors.