A booking reference (also called Passenger Name Record, or PNR) is a 6-digit alphanumeric code that can be found on your booking confirmation and any emails you received from the airline or travel agency regarding your trip. All airlines require this code prior to accepting a claim for compensation.
You can find your booking confirmation in your email correspondence with the airline and/or your travel agent. If you think you may have deleted this email, try searching for the airline name or the website you used to purchase your ticket in both, your inbox and trash folders.