eticket
If you received an eticket via email, your booking reference is the six character alphanumeric code that appears after the 'BOOKING REFERENCE' field.
eticket (airline printed receipt)
On your flight coupon / ticket, your PNR is the first six alphanumeric characters printed toward the right as shown in the illustration below.
An eticket from a travel agent will also contain another six-character booking reference – you can also use this to check in online at emirates.com