I'm pretty sure the person asking the question is not asking how to eject a disk with it's image mounted on the desktop.
Quick way: Go to Applications/Utilities/Disk Utility and choose First Aid. You'll see your drives, including the CD/DVD drive. You will see the disk that is stuck in there, but it will be grayed out. Click on it to select it. See the eject button above the list? Click that and the disk will pop out.
A disk that isn't even recognized by the Disk Utility can be dealt with by using Terminal. Launch Terminal, located at /Applications/Utilities. In the Terminal window, enter one of the following three commands: If you have a single optical drive: drutil eject
If you have both an internal and external optical drive, use the appropriate command below, depending on which drive has the stuck CD or DVD:
drutil eject internal or
drutil eject external
Press return or enter after entering one of the above commands in Terminal. The stuck CD or DVD should be ejected.