Au Pair is a long term visa and comes under category D National visas. Basically for applying for au pair visa, au pair candidate will need to set up an appointment in advance at the German Embassy.
And visit the German Embassy on the specific date and on time along with all the required documents and visa fees of 60 Euros as mentioned in the link below:
Visa Information for Au Pairs
Upon arrival in the consular section, the au pair is interviewed by the consular officer. If your visa application is successful, you will be called within a week along with your passport for the collection of your visa.
For more information regarding being an Au Pair in Germany, candidates are requested to visit the following webpage: