I agree with LadyMuck - assume the au pair knows nothing - you will have to teach her from scratch. She probably won't have a clue - I have had to tell mine not to leave the children alone in the bath, I have had to show her how to cook sausages, I have had to show her how to do everything I assume people know how to do. But when I was 18 I was very similar!
I do find it a bind having to cook for her, but get round that by giving her what the children have (I don't usually eat proper meals in the evening, and if I do it is much later with DH).
I prefer mine not to sit with me in the evening, but you can't tell them not to! Just make their room as inviting as possible with a tv, dvd, stereo etc.
Write a very detailed list of duties (mine takes the form of a timetable) - assume that she won't do anything unless it is in the list - you will probably be pleasantly surprised.
~Remember that often the au pair is not more than a child and might be away from home for the first time. Mine is 19 and I feel that I have to be responsible for her too. I don't mind this, but if you do, maybe you should go for someone older.
There are lots of plusses from having an au pair. Your's will probably be able to go and do the school run while your dh stays with the baby. It is nice watching the children form a relationship with the au pair- My older daughter really likes ours - they do each other's hair!
I went through an agency to find my latest au pair - but the best way is a personal recommendation. my last au pair was recommended and stayed with me for 18 months. We are still in touch. The baby was extremely fond of her (she was 6 weeks when she started).
If you have any other questions, I am happy to answer them (if I can!).