You have three ways to book your bike for flying.
Many, especially low cost airlines, offer the booking of sports goods like a bike directly in the same booking process like you book your own seat. Other airlines are asking for a telephone call to reserve the bike. There a even some operators who are not interested in you bike before you come the airport because they use big planes and have enough space.
Online, on the phone or on-site - the bicycle must be booked, too.