How to find your Service Tag:
Desktops: Your Service Tag is a unique five- to seven- digit alphanumeric (letter and number) code, which is found on a white bar-coded label affixed on the back or on the side panel of your Dell computer or peripheral.

(image from support.dell.com)

Notebooks: Your Service Tag is a unique five- to seven- digit alphanumeric (letter and number) code, which is found on a white bar-coded label affixed on the bottom of your Dell computer or peripheral.