How do I make a script that puts a tool in your inventory based on a leaderstat?

This is not that hard to make and achieve

First to make this easier you should make a reward dictionaries for example:

local rewards = {
    [100] = "Your sword name here",
    [500] = "Your sword name here"

    -- you can add as many as you want
}

After that you want to reward the player with :GetPropertyChangedSignal. If you use DataStore this script will also work because upon loading data your value will get change if I am not wrong

local SS = game:GetService("ServerStorage")
local rewards = {
    [100] = "Part",
    [500] = "Your sword name here"

    -- you can add as many as you want
}

local function giveTool(player)
    for requirement, reward in pairs(rewards) do
        if tonumber(requirement) <= kills.Value then
            local hasSword
            hasSword = player.Backpack:FindFirstChild(reward)

            if hasSword then
                return
            else
                local newSword = SS:WaitForChild(reward):Clone()
                newSword.Parent = player.Backpack
            end
        end
    end
end

game.Players.PlayerAdded:Connect(function(player)
    -- your simple leaderstats script here
    local leaderstats = Instance.new("Folder")
    leaderstats.Name = "leaderstats"
    leaderstats.Parent = player

    local kills = Instance.new("IntValue")
    kills.Name = "kills"
    kills.Parent = leaderstats

    kills:GetPropertyChangedSignal("Value"):Connect(function()
        giveTool(player)
    end)

    player.Character.CharacterAdded:Connect(function()
        giveTool(player)
    end)
end)

Note: You might need to check if player is holding a sword to prevent multiple rewards Edit: As elyz said below, I edited it so it gives you the sword after dying. Just look at my answer as a sequel for multiple rewards.

like AntoninFearless said, you can use the .Changed event to detect when a player reaches a certain amount of kills and give them the sword

keep in mind that the player's backpack is cleared when they die, so you'll need to give them the sword when they respawn too

local swordKillRequirement = someNumberHere -- you should probably use a table for this if you're going to add lots of different swords, but for just one sword a variable is enough
local sword = path.to.Tool

Players.PlayerAdded:Connect(function(player)

    -- blablabla leaderstats stuff goes here

    leaderstats.Kills.Changed:Connect(function(kills) -- for Values, the .Changed event only fires when the value changes, unlike other types of instances where it fires for every property change
        local tool = player.Backpack:FindFirstChild(sword.Name) or player.Character:FindFirstChild(sword.Name) -- tools get parented to the character when they're equipped, so we need to check in the character too
        if kills >= swordKillRequirement and not tool then
            sword:Clone().Parent = player.Backpack
        end
    end)

    -- add the sword when the player respawns
    player.CharacterAdded:Connect(function()
        if leaderstats.Kills.Value >= swordKillRequirement then
            sword:Clone().Parent = player.Backpack
        end
    end)

    -- datastore stuff goes here, if you have it

end)