How to save/load this IntValues?

I highly recommend you check this out: http://wiki.roblox.com/index.php?title=Saving_Player_Data

Here's the script with some explanations: (Make sure your game has access to API Services, or else DataStore won't work)

DataStore = game:GetService("DataStoreService"):GetDataStore("Stats")--Name whatever you want
local GoodBoy = script.EXPNeeded.Value
SaveTime = 60 -- Saves every (SaveTime) seconds (I recommend not going under 60 seconds...)

game.Players.PlayerAdded:Connect(function(plr)
    local PlayerKey = "user_".. plr.userId -- Sets a key for the Player's stored data
    local stats = Instance.new("IntValue", plr)
    stats.Name = "leaderstats"
    if DataStore:GetAsync(PlayerKey)then --This function loads data with the Player's key. If there is no data, then it will retrun nul. So here we are checking if there is data
        print("Loading " .. PlayerKey)
        local Data = DataStore:GetAsync(PlayerKey) -- Loads our saved Table
        print(Data.Souls)
        local EXP = Instance.new("IntValue", stats)
        EXP.Name = "EXP"
        EXP.Value = Data.EXP
        local level = Instance.new("IntValue", stats)
        level.Name = "Level"
        level.Value = Data.level
        local Souls = Instance.new("IntValue", stats)
        Souls.Name = "Souls"
        Souls.Value = Data.Souls
    else do -- Setups new data
        local NewStats = {EXP = 0,level = 0,Souls = 0,} --This Table saves all our Variable's Data
        local EXP = Instance.new("IntValue", stats)
        EXP.Name = "EXP"
        EXP.Value = 0
        local level = Instance.new("IntValue", stats)
        level.Name = "Level"
        level.Value = 0
        local Souls = Instance.new("IntValue", stats)
        Souls.Name = "Souls"
        Souls.Value = 0
        DataStore:SetAsync(PlayerKey,NewStats) -- This saves our Table
    end
    end
plr.leaderstats.EXP.Changed:Connect(function() --Not sure what this is... but it's in your original script sooo I left it here...
    plr.leaderstats.Level.Value = math.floor(plr.leaderstats.EXP.Value/script.EXPNeeded.Value) --Added .Value
    plr.leaderstats.EXP.Value = 0
    GoodBoy = GoodBoy + 10 --Removed script.GoodBoy
  end)
while true do --Save loop
    wait(SaveTime)
    local PlayerKey = "user_".. plr.userId -- Sets a key for the Player's stored data
    local Stats = plr.leaderstats
    local Data = {}
    Data.EXP = Stats.EXP.Value
    Data.level = Stats.Level.Value
    Data.Souls = Stats.Souls.Value
    DataStore:SetAsync(PlayerKey,Data)
    print("Saved Data")
    end
end)

game.Players.PlayerRemoving:Connect(function(Player) -- This will save the Player's data when leaving the game
    local PlayerKey = "user_".. Player.userId -- Sets a key for the Player's stored data
    local Stats = Player.leaderstats
    local Data = {}
    Data.EXP = Stats.EXP.Value
    Data.level = Stats.Level.Value
    Data.Souls = Stats.Souls.Value
    DataStore:SetAsync(PlayerKey,Data) -- Saves our Data Table
end)

I also recommend you read through here: https://wiki.roblox.com/index.php?title=API:Class/GlobalDataStore (That has everything you need to know about Datastore.)