This depends on the type of airline you are flying with. There are 2 options:
1. Low-cost airline (such as Ryanair, Transavia, easyJet and Wizz Air)
On the confirmation email you received immediately after booking, your booking references are clearly stated. All details can be found in this e-mail.
2. Other airlines
After booking, you will first receive an email with the booking confirmation. You will receive your ticket via email within 22 business days (but no later than 24 hours prior to departure). If you are booking within 24 hours before departure, you will receive your ticket via e-mail immediately. Your ticket e-mail consists of a booking reference and/or ticket number. With these references you are able to check-in at (the website of) the airline in order to receive your boarding pass(es). Click here for more information.
Make sure to also check your spam inbox. Did you not receive or lose your ticket? Log in on MijnTix and go to ‘Your Booking’ and click 'Travel Documents' and download your ticket e-mail.