Imac dilemma

Ah, this is my first post in the Macintosh area. I'm almost always working on PCs for resale but I have an older Imac that I've got problems with. Apparently, the operating system isn't on the harddrive or has been corrupted as I only get the flashing "mac guy/question mark" folder in the center of the screen when I turn it on. I found a CD that I thought might be the OS or a boot disk and put it into the drive. It's not the right disk. How can I get it out? There is no "eject" button that I can find. Is there anyway to get this out and can someone give me a little background on this machine? I've used many macs at print shops but have rarely needed to work on them. Thanks!