I changed computers from a pc to a iMac. How do I sync my IPad to the new computer?

iTunes: How to move your music to a new computer - Support - Apple

Do you still have the old computer? If you do, you can transfer the iTunes library from the PC to the Mac. You will help here.

http://support.apple.com/kb/HT4527

If you no longer have the PC, you must transfer the contents of the iPad into iTunes before you can sync with it. This user tip will describe the process to you. It was written for an iPhone/iPod, but the process is the same for an iPad.

https://discussions.apple.com/docs/DOC-3141