looking for serial number

In the  menu ➙ About this Mac window:

If you're on a Mac, go to the top left and click on the Apple Logo < About This Mac.

If you're on an iPhone or iPad, go to Settings < General < About

Move your mouse to the top-left of your screen and click the apple logo, then click "about this mac." A window will appear, displaying your serial number, among other things.

If the computer does not work, and your iMac is a model with a metal "foot' that holds the display, the serial number on on the bottom side of the foot.

You don't have Mac OS installed?

As mentioned, power off the computer and turn it upside down to see the number on the bottom of the stand/foot.

bhaimsohn wrote:

I am running windows on my mac. There is no apple logo in upper left corner of my computer screen.

In cmd running as admin, type:

wmic bios get SerialNumber

And press enter