If you have booked your ticket on qatarairways.com, you can select your seat from the booking confirmation page upon completion of your booking. 

You can also set your seat preference through "Manage Booking" on qatarairways.com. Enter your last name and booking reference number to retrieve your booking, then select "modify seat selection" to choose your preferred seat. 

In addition, you can select your preferred seat when you check in online, up to 90 minutes prior to your flight's departure. 

Qatar Airways reservations agents and travel agents will also be able to book preferred seats.