IMEI is a 15 digit number, which can be displayed on phone LCD by dialing *#06# just as a phone number.

Please enter only numbers, without spaces or signs like "-" or "/". Quite often the IMEI number can be longer, but we only need the first 15 digits.
If you own an Apple product like iPhone or iPad, you can find the IMEI number in the phone settings. Android devices also have an IMEI number in the phone settings.
Other devices like modem, which doesn't have a screen, need to check the sticker on the device to find the IMEI number.