iPod Nano 4GB from PC to iMac

I have an iPod Nano 4GB that was formatted for Windows a couple of years ago. Shortly afterwards I bought and iMac and would like to like to be able to sync my iPod with the iMac. Most songs were loaded from CDs. However, a few were purchased from iTunes on the PC. What is the best way to move the entire iTunes library from the PC to iMac and allow synching between iPod and iMac going forward? Thanks.