How do I get support for my iMac? Go to > About This Mac. The serial number is listed. You can also find the serial number on the bottom of the iMac stand.
How do I know which model of iMac I have? See the Apple Support article Back up and restore your Mac.
What should I do before selling or trading in my Mac? Back up your data, then restore your Mac to factory settings. See the Apple Support article What to do before you sell, give away, or trade in your Mac.