How would i tell if a player joins for the first time?

If this works +1 and accept please

Ideaology

So you want to check if it's the players first time joining eh? Well the ideaology behind it is saving a value this value could be an int,string,bool but as long as it's there it shows that the player has been there. For server checks that stay in the server it could be as simple as making a model and checking if theres anything in there saying that player has joined.

modelcheck = game.Workspace.ModelCheck
game.Players.PlayerAdded:connect(function(player)
    if modelcheck:FindFirstChild(player.Name) == nill then
        check = Instance.new('BoolValue')
        check.Name = player.Name
        check.Parent = modelcheck
    elseif modelcheck:FindFirstChild(player.Name) ~= nill then
        print("Player's not first time here")
    end
end)

Ideaology for the entireity of your game

So, server for the entire game is a little different, you would need to save the value over all your servers. For this you would use a form of data persistance I usually use data store.

local DataStore = game:GetService("DataStoreService"):GetDataStore("Points")
game.Players.PlayerAdded:connect(function(player)
    key = player.Name
    if DataStore:GetAsync(key) then
    print("Player Exists")
    else
    print("Player Doesn't Exist")
    DataStore:SetAsync(key, true)
end
end)