When you book a flight, baggage is not included unless clearly specified otherwise. Luggage depends on the fare type that the airline provides, therefore it is stated clearly when it is included. For some airlines you have the option to add luggage/baggage in the booking journey online.

If this option is not available online and you wish to add luggage/baggage to your trip, you need to do so after the completion of your booking by logging into your account on the airline's website, if this option is not available due to code share or other reasons, this will need to be done at the airport, where extra costs apply.