When I got my first au pair job, there wasn't any resource to help me out. I had to figure out everything on my own. I think the most important thing to be a happy au pair is the respect between the au pair and the host family. You and the family live in the same house like most of us live with our parents now. We have to obey the rules of the house and live as a part of the family in that house with the host family. Our parents have rules in the house, calling in if we will be late or cleaning up after meals and doing things around the house. In return, the host family should treat the au pair as a part of the family and include the au pair in activities and such.
Another important thing is communication. If there is a problem, it should be resolved by talking to each other. This is the only way to let each other know if something is bothering either the au pair or the host family. Do not let anything bother you for a long time becase it would only get worse if you don't take care of the problem when they arise.
Keep in mind that you should take your time when finding a host family. Most of us find our host families online. There are many web sites offering services to find au pair jobs. My favorite web site for this is http://www.aupairconnect.com because I can find families from all over the world. I also had very good luck with the families I found there. For a small fee, you can email any family you like for 3 months. If you can't pay for the membership, you can let the the families know that you are interested in them when you add them to your favorite lists. Then they can contact you.
Make sure you correspond with them and talk about everything, their lifestyle, children's habits, what they expect and tell them what you expect from them. If you want to go to a language school, make sure they locate one before you sign up for the job. Make sure you can get to the school by bus, bike or car (if provided and can drive) and your schedule will allow you to go to the school.
Some things might seem difficult when aupairing in another country, homesickness, a new language, being away from your family. However, I am very glad I was an au pair. It was one of the best things I have done. I learned new languages and made new friends. I gained more experience with children. I keep in touch with my host families and they send me the pictures of the children every year. I would recommend it to anyone.