I'm trying to change computers but iTunes will only download purchased music. How do I go about syncing my iPod so that it will get my entire library on the new computer?
You need to copy everything from your old computer, or your backup copy of your old computer, to the new one.
The music sync is one way - computer to ipod. The exception is itunes purchases.
if you have an external hard drive follow this article
Backing up iTunes Library
Retrieving data ...