Hi and welcome to Just Answer!
My name is Jamie.
The ESN or MEID number are located under the battery on the phone itself. There will be a sticker on the phone that lists these numbers.
With an MEID it's a 14 digit letter/number combo with the first letter always starting with the letters A-F.
With an ESN it's an 11 digit number with an 8 digit hex directly below. You will need both numbers usually.
If you need anything else please let me know!