Q: how do I move music copied from a CD onto my iPad onto my new computer?

Copy it from the old PC to the iMac. 

There are a variety of techniques including Migration Assistant for Windows, enabling file sharing between the two computers, or copying your music to a flash drive or other storage device followed by importing it to iTunes.

iTunes: How to move your music from one computer to another

Miragtion Assistant:

OS X Mountain Lion: Transfer items from a Windows computer to a Mac

On the Windows machine, download Windows Migration Assistant here: Windows Migration Assistant v1.0.1

Instructions are here: http://support.apple.com/kb/HT4796

Once your music library and everything else you want to transfer from the old PC is on the iMac, sync your iPad to the iMac:

You will choose Erase and Sync which will do exactly that, in that order.