One of the most important things you should decide first is where you are hoping to au pair. Your application/profile will most likely ask you to narrow down your choices- and it's best you do so specifically.
Rather than just putting Europe, list the countries you are specifically interested in. Also, specify what type of location you would live in. For instance, I put "big to medium sized cities." This is important while "village" and "rural town" mean just that- you and one other family within two hours of a larger city. (specifics are key.)
Note: If you suggest you would like to live in a big city, do not assume you will live in cities like Paris, Berlin, London. Most likely, you will live in a city outside of the big cities (which is awesome in its own self.)
However, don't limit yourself. If you are contacted by a family outside of a city you were hoping to work in, check out google maps and investigate commute times and stay open-minded. I live in a beautiful ocean-side city and I love being 30 minutes outside of the big city. It's always nice returning to our little seaside city for daily life after a weekend out in the busy hub-bub of the bigger city.
If there is another thing you definitely need to do before choosing a place to call home for a year, it's do your research. I personally had studied German since junior high and had taken many courses on intercultural communication in college, so I felt pretty confident about what to expect. However, I have learned a lot of new things since being here and find myself google-ing certain cultural mannerisms frequently.
Dive into anything and everything you can get your hands on about a country/culture you are interested in. As soon as you think you're pretty literate about most of the cultural norms you find yourself in many everyday situations you hadn't even thought about being different and thinking: "Holy moly. what am I supposed to do!" (but hey, moments like that keep you on your toes & adventures around every corner.)
Take the following things into consideration, for instance:
expectations of children
Of course, I'm not suggesting you generalize a culture altogether- rather, encouraging you to get an overall sense of the cultural differences.