Starting having problems with the internal hard drive in my early 2008 20 inch iMac. Decided to replace the disk with a solid state drive (ssd). To get into an iMac, one of the first things you have to do is remove the glass screen, which is held in place by magnets.
The suction cups were ideal for removing the screen. Just apply the cups and gently lift straight out to the chassis.
Spend an hour or so viewing videos and written descriptions of how to take apart an iMac. It's really easy if you understand what you have to do.