If you have Toast, just go to the Copy tab.
Without Toast, you'll have to use Disk Utility. Insert the CD and open DU. On the left panel, click the session you want to copy (usually there will be 3 CD icons, and you normally want the middle one). Go to Images -> New -> Image from CD Name (the last option). I recommend using DVD/CD master and no encryption.
Once it's finished, you can use the Burn toolbar button to copy the image file onto a new CD.