There are a few ways to transfer your iTunes library from one computer to another:
When you get a new Mac, use Setup Assistant (or Migration Assistant, if you’re switching from Windows) to move files—including your iTunes files—from your old computer to the new one.
Connect computers (over a wireless network or with a cable) and copy your iTunes files from one computer to the other.
Use iPod or a portable drive to transfer iTunes files.
Before you sell or give away a computer, don’t forget to deauthorize it (choose Account > Deauthorize Computer).