The idea of the forums is to help with problems not to do the homework or assignments for you

Start with the easiest one - saveFlight() your assignment says save the details in a file called Flight.txt. SaveFlight will be passed a Flight object, it has getters that will enable you to retrieve the state of the object, you then store these values in a file using java.io ( or if you have seen it yet serialisation) How you store them is up to you, maybe just read each value and write it to the file in turn. You can check if it works by looking at the file. Once that work you should be well on the way to savePassenger and saveReservation as they are similar.
Shout if you get problems and post code that is causing you problems.