IMEI number also known as serial number is a special 15 digit number which can identify your device.
It usually looks like this xxxxxxx-xx-xxxxx-x or xxxxxxx-xxxxxxx-x.
In order to check your IMEI number, just dial *#06# sequence, like a phone number.
If you have a device with a touch screen, go to calling menu, and enter *#06#.
Check the link below to see a video help for the instruction.