In general, you can book seating after you have made your flight reservations. However, certain airlines may not permit seat selections for fare type booked (e.g. promotional fares). Most airlines now charge a fee for seat selection.
If you do not wish to pay the seat selection fee, you may select your seating for free when the airline opens the flight for online check-in (usually between 36-48 hours before departure) except for low cost carriers which the seating will be randomly selected, unless a seat fee is paid.