If online check-in is not an option, you can always check in at the airport (note: in some cases an additional fee applies). Check-in can involve long waiting times, so always make sure you arrive at the airport in good time.
Take your luggage with you to the airport on the day of departure. If you have booked checked baggage, take it to the baggage claim or check-in desk. Some airports also have very convenient self-service check-in machines. After this, you proceed to the security check. Your boarding pass will be scanned/checked, so make sure you have your boarding pass in paper or mobile format close at hand. In most cases, you will need to pass through customs to check your passport.
If you have time to spare, you can also shop at the airport after passing through passport control, but make sure you arrive at the gate in good time. The screens at the airport show flight information (the gate number and boarding time of your flight). If you miss your flight, your luggage will be unloaded and you will usually have to book a new flight ticket.