How to make in that script random item from lighting?

You could do it using math.random(), like so:

local debounce = false

local function getPlayer (humanoid) -- You can (and should) localise functions, too.
    local players = game.Players:children()
    for i = 1, #players do
        if players[i].Character.Humanoid == humanoid then return players[i] end
    end
    return nil
end


local function onTouch(part)
    local human = part.Parent:FindFirstChild("Humanoid") -- findFirstChild is deprecated; use FindFirstChild instead.
    if (human ~= nil) and debounce == false then
        debounce = true
        local player = getPlayer(human)
        if (player == nil) then return end
	local weapons = game.Lighting:GetChildren()
	-- Note the changes here.	
	weapons[math.random(1, #weapons)]:Clone().Parent = player.Backpack
        wait(5)
        debounce = false
    end
end

Also, you might want to put the weapons inside a Folder inside ReplicatedStorage or ServerStorage instead of putting weapons in the Lighting.