From your OLD computer...
Copy your ENTIRE iTunes FOLDER to an External Drive... and then from the External Drive to your New Computer..
Full Details Here > http://support.apple.com/kb/HT1751
If your Old Computer is a PC... See Here...
Move iTunes Library from PC to MAC
Welcome to the Apple Support Communities
Instead of transferring only your music, you can transfer the whole library, so it will be exactly the same you have on the PC. Follow these steps > http://support.apple.com/kb/HT1751?viewlocale=en_US&locale=en_US
You have to do on the Mac the "Restoring your iTunes Library backup" step