Well hey there HwaJong!
It's an absolute pleasure to see you here in the Apple Support Communities! How are you supposed to kickstart your day without your tunes? Not to worry though, we can work to get those back. Congratulations, by the way, on your new MacBook Air! That's my personal favorite Apple Device.
To start, have a look over this resource which will help you download your past purchases:
Download your past purchases
Enjoy that new computer!