If you still have access to the Computer you used to Sync with you need to Transfere the iTunes Library from it to your New one...
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
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
Go into iTunes and change the setting for automatic syncing. This will prevent iTunes wiping your phone clean. Do you have music/apps on the phone? If so, where is that content from your library from the other computer? It sounds as if this is not the computer you used to sync the phone to, am I correct? You can backup the phone, but just remember you will need to transfer purchases to move apps and music that is on the phone and no in iTunes over to iTunes. What about contacts and calendar, where are they located? Anywhere but the phone? This is why computers need to be downloaded so you don't lose content. Check this support document https://discussions.apple.com/docs/DOC-3141