You can pre-book your seats by visiting your airlines website and clicking on the manage booking section.
You'll be able to choose your seats from the airline seat map and pay the airline directly. Login details vary by airline but in most cases you’ll need your Airline Booking Reference which you can find in the Flight Voucher on your booking confirmation paperwork or by logging into My Bookings. See more information on how to find your airline booking reference here.
Just a heads up - any pre-booked seats you add directly via the airlines website won't show up on your On the Beach confirmation paperwork or in My Bookings - don't worry, though - the airline will have everything they need on your flight booking.