how can i save when a player first joined?

i want to save when a player first joins and then a gui pop's up, and when they join back again the gui doesn't pop up. anyone know how to do that? i was gonna use DataStores but i dont know how to use them that well, please help me thanks.

local DataStoreService = game:GetService("DataStoreService")
local ReplicatedStorage = game:GetService("ReplicatedStorage")
local Players = game:GetService("Players")

local JoinData = DataStoreService:GetDataStore("JoinData")
local JoinPopup = ReplicatedStorage:WaitForChild("JoinPopup")

local function CheckIfJoined(Player)
    local Success, Response = pcall(JoinData.GetAsync, JoinData, Player.UserId)
    if (Success) then
        local Joined = select(1,unpack(Response))
        if not (Joined) then
            JoinPopup:FireClient(Player)
            SetJoinData(Player)
        end
    else
        warn(Response)
    end
end

local function SetJoinData(Player)
    local JoinInformation = {
        Joined = true;
        Date = os.date('%x');
        UnixTimeStamp = tick()
    }
    JoinData:SetAsync(Player.UserId, JoinInformation)
end

Players.PlayerAdded:Connect(CheckIfJoined)