iMac wont boot with cdrom

First of all, this is my first time working with a mac. I want to reinstall my iMac to remove the pre-installed OS. I boot up and hold ALT to enter the boot menu. But there I can only choose the harddrive to boot from, thats the only option. The cdrom drive isnt showing. I have an original Tiger OS cd inserted.
What can be wrong and how could I fix this? Thx..

is it a retail copy of tiger or the grey cd that came with your mac

try holding down the C key on start-up

is it a retail copy of tiger or the grey cd that came with your mac

try holding down the C key on start-up

the cd is not grey, it must be retail.
ive tried so many key combinations during startup. ALT is the only one showing any kind of boot menu.

To see the boot menu press Option key on your Mac KB.

To boot X (to boot Mac OSX) regardless, fire up the computer and hold down the X key. Even if the HDD is not being seen the computer should boot from the first bootable volume it finds, and if you have the Mac OSX CD in the drive it should see it and boot. I found this happened when the system didn't recognize nor mount the HD I had installed.

Alternatively, try and obtain a gray CD appropriate for the model you are trying to boot. The reason the retail CD (or DVD) does not work is that each machine may require certain drivers which are not available on the Retail CDs.