If an ESN is provided, it can only be converted between Hexadecimal and Decimal formats. It cannot be turned into an IMEI unless the device is a "World Phone" containing both radios, in which case the ESN and IMEI are merely co-located in the database, not mathematically derived from one another.
You can find your MEID in your device Settings menu or behind the battery if you have a removable one. esn dec meid converter to imei fixed