First, you must be registered as an employer with HMRC. Upon registering you will receive a welcome pack which will include your ERN and confirmation of your registration. Upon receipt of this letter, you should store it in a safe place.
If you can no longer find your documentation that includes your ERN, your next port of call would be to take a look at any tax forms/reminders or payslips. You will also find this information on any P45 or P60 documents.