They might ask, "Did you make a reservation?" In this scenario, you didn't so you can answer, 'no.' If they don't have any cars available, they will say, "I'm sorry, we do not have any cars available today." If that is the case, then go to the next counter. Most airports have numerous car rental booths so you can keep asking until you find a company that has a car available. If you are going during peak vacation periods, it might be good to call and reserve one. But let's continue with this scenario.
If they have a car available, they will ask what type of car or what size.