For airlines that allow seat selection, you can choose a seat during the booking process. On the flight search page, click anywhere on the flight card to expand it. You'll see more details appear including a "Seat Map" link.
Click the Seat Map link to see your options for that particular flight. Hovering over seats will reveal extra information such as type and price. After selecting your seat, click "Save" and return to the search page. When you're ready, click the "Select" button to choose a flight.
You can also choose your seats from a Draft Trip. Find your draft trips on the Trips page by selecting the "Drafts" tab. Click on a trip card to see the itinerary. Then choose the "Edit" button. This will return you to the flights search page, where you can follow the above instructions to change your seat assignment.
Although the seats selected were available when you started the booking process, there is a chance a seat was assigned to another customer prior to the completion of your transaction. If seats are unavailable on the displayed seat map, you can obtain a seat assignment upon check-in for your flight.