How can you save everything(that you have done in a game) till the next time you join?

I'll cover you up.

What you want is called "DataStores"

That can be done this way:

Insert a script in the workspace, and write this:

--Get 3 services (yah, a lot of stuff going on here)
local DataStoreService = game:GetService("DataStoreService")
local Players = game:GetService("Players")
local ReplicatedStorage = game:GetService("ReplicatedStorage")
 
--The player's money will go here.
local playerMoneyStore = DataStoreService:GetDataStore("PlayerMoneyStore")
 
--The notification event goes here.
local playerNotificationEvent = Instance.new("RemoteEvent")
playerNotificationEvent.Name = "PlayerNotificationEvent"
playerNotificationEvent.Parent = ReplicatedStorage
 
--Create the playerMoney table.
local playerMoney = {}
 
local function onPlayerAdded(player)
	--get player's id.
	local playerId = tostring(player.UserId)
	--if successful, then return playerMoneyStore, defined a bit ahead.
	local success, money = pcall(function()
		return playerMoneyStore:GetAsync(playerId)
	end)
	if success then
		playerMoney[player] = money
	else
	--Say there's an error on saving data.
		playerNotificationEvent:FireClient(player, "Error retrieving saved data: please try again later")
	end
 
	-- Do other player initialization code
end
 
Players.PlayerAdded:Connect(onPlayerAdded)

See if that works.

As always, good scripting