Can I choose my seat?

On many flights, yes. When it's offered, you'll see the option to pick a seat during booking, and the price (if any) is shown clearly before you pay.

Seat selection is a paid extra on some airlines and free on others; it also varies by fare and by where the seat is on the plane. If it's not available at booking, you can often still choose a seat later with the airline using your PNR.

If you don't select a seat, the airline assigns one automatically, usually at check-in. Travellers on the same booking aren't always seated together by default, so if sitting together matters, choosing seats is the safest bet.