With most phones, the ESN (or MEID) is at the back of the phone, underneath the battery. The ESN/ MEID is normally an 18-digit number labeled DEC. On some older phones, the ESN/MEID is an 11-digit number.
If your phone does not have a removable battery, or if the ESN/MEID is not located underneath the battery, you can discover it through your phone settings (most likely in the “About Phone” menu).