This varies per airline. Often, you can check in online up until 24 hours before departure, but sometimes you will even have more time to do so. The exact check in time can be found on the website of the airline.
Not checking in online? Make sure you arrive at the airport well before departure. You can then check in at the check-in desk.
ATTENTION: Some low-cost airlines (such as Ryanair, easyJet, etc.) may charge additional fees when checking in at the airport.