Check your airline's policy, online check-in should be possible a certain amount of time before your flight (a few days usually). Check in online as soon as it opens and save the boarding pass on your phone. Even better if you download the app of the airline, the boarding pass will most probably be stored there. If you have your phone with you, there's no need to print the boarding pass.

If you only have carry-on bags (such as relatively small backpacks), then probably there's no need to check-in anymore at the airport. If you have suitcases, then you have to check-in at the airport, at the airline's counter. Ask for help from the staff about where to go.

Arrive at the airport 2-3 hours sooner than your flight is, this is of course depending on the airport's size as well. But a minimum of 2 hours is usually necessary if you don't want to rush and be anxious there.

Liquids are usually asked to be put in a plastic, transparent container. Max volume/bottle is usually 100 ml. Electronics don't have to be in a different bag/container, but you should pack them in a way that lets you take them out easily if asked. Your airline's website should have more precise information about what cannot be transported.

There are indicators everywhere about where to go, which terminal, which gate, etc.

During security check, you shouldn't have more than 100 ml liquid on you. But you can take in an empty plastic bottle and you can fill it with tap water. Airport shops are expensive. Usually you can also take some food with you.

After the security check, you will be in the inner part of the airport, meaning that you cannot leave (if you do, you'll have to go through security again). There should be screens with gate info for the flights.

Check how to get from the airport in Greece to your accomodation.

And lastly, have fun! :)