I always suggest sending an email first as an introduction and asking a good time to call. Some au pairs do not have regular email, so if you don't hear from her, call anyway. It is nice to tell the au pair some about your family (since she is limited as to the information she has gotten) so that she can ask some appropriate questions about you.
Why do you want to be an au pair?
Will this be your first time away from home? Do you think you will feel homesick?
What will be the most difficult part of your year?
What do you do in your free time?
What are your plans when you return home?
What are some of the things you want to do while your here?
What age children have you cared for?
What were your responsibilities?
What do you like most about caring for children? What do you like least?
What will you do if the children won't listen?
How long have you been driving? Where? How often? How do you feel about driving in this area?
What would you do if the baby is sleeping?
What activities do you do with my age children?
What activities would you do if it were raining?
How would you react if my child says "But Mommy and Daddy let me do it"?
Also think of things that are specific to your children, family and area. If there was something on the application that gave you hesitation or you weren't sure about, ASK.