How can I run this when the player joins the game OR gets a new item?

So, I've tried multiple ways of doing this, but it normally just overlaps or duplicates the previous items, so how can I run this when a player either joins, or gains an item (plr:WaitForChild("Inventory").ChildAdded)

local fram = script.Parent.back
local plr = script.Parent.Parent.Parent
function UpdateOldLabels(Parent)	
	for i,v in pairs(Parent:GetChildren()) do
		if v.Name:sub(1,4):lower() == "item" then
			local LineNumber = v.Name:sub(5)
			v.Name = "item"..tostring(tonumber(LineNumber) + 1)
			v.Position = v.Position + UDim2.new(0.02,0,0,0)
		end
	end
end

for _,v in pairs(plr:WaitForChild("Inventory"):GetChildren()) do
	local item = Instance.new("ImageButton", fram)
	item.BackgroundTransparency = 1
	item.Name = "item1"
	item.Size = UDim2.new(.02, 0, .6, 0)
	item.Position = UDim2.new(-.02, 0, 0.15, 0)
	item.Image = "rbxassetid://"..tostring(v.itemPrev.Value)
	item.ZIndex = 2
	UpdateOldLabels(fram)
end

I believe it may be if game.Players.PlayerAdded then end