how can i make a gui pop up once when the player joins for the first time?

hi! i want to make a script that when a player first joins the game they get a gui saying "Welcome" and then when they leave and join back the gui doesnt come back, i dont know how to do this i was gonna use datastores but this is my first time using them and i dont know how to use them at all and i dont know how they work. i made a small script here:

local data = {}

local datastore = game:GetService("DataStoreService")

local MainData = datastore:GetDataStore("FirstTimeLoaded")

game.Players.PlayerAdded:Connect(function(plr)
	
	local succ, errormsg pcall(function()
		MainData:SetAsync(plr.UserId.."//new user\\", plr)
	end)
	
	if succ then
		print('worked')
	else
		print('Error While Loading')
		wait(errormsg)
	end
end)

i want it to get the data from the table in local data = {} and see if the player has joined before. please help me thanks.