If you do not have any previous travel agency experience try and find out or think of a personal trip you might have assisted in planning, i.e. A weekend break to New York with internal flights to LA and a stop off visiting relatives etc. At least this shows to the interviewer that you have some sense of how gathering together an itinerary works. For the more experienced, add the more specific tasks i.e. selling the tour packages by choosing the right destination with lots of excursions and day tours, planning appropriate transportation and accommodation, firm up travel dates, make detailed itineraries and sell these tour packages. Resolving any clients' travel issues.