The iMac
There is no apple key on a newer iMac.
Yes, with the built in application Garage Band.
The iMac is an Apple computer.
You do not make the iMac, Apple does.
The Apple iMac is a computer and its purpose is to compute.
One can read Apple iMac reviews online at various websites. One can read Apple iMac reviews online at websites such as CNET, TechRadar, and Computerworld.
If you are downloading it from the Apple App Store you must create an Apple ID there.
The Apple iMac comes with iChat Instant Messaging software. You can download other IM software such as Microsoft's Messenger or Adium (See links below) which will connect to many IM services.
I do not have a 24" iMac. I have a 20" iMac. Apple no longer sells the 24" iMac. If you will click the Apple logo on the top left of your screen and then click About This Mac then More Info you can find the answer.
Apple
Apple