If you still have access to your Old computer that you used to Sync to...
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
An Added Bonus is that you will have a Backup of iTunes.
If not... See these 2 Links...
Syncing to a New Computer...
Recovering your iTunes library from your iPod or iOS device