To be a legal au pair in the USA with a J-1 visa, au pair and the family must register themselves with one of the agencies. Note that both of you must be registered with the agency that work together.
Agencies provide the documents that are needed for a J-1 visa and do a background check on the au pair. Au pair applies for a J-1 visa at the US embassy or consulate. This visa is valid for 1 year. You can also find a great deal of information about the aupair program at the web site of US Department of State.
Agency fees for the family are about $5000-6000. Au pair receives $139 a week pocket money. Educational assistance of $500 is often required as au pairs need to attend some classes in the US. Au pair must deposit some money to the agency in her country, it is refundable if the au pair successfully completes the program. This fee is around $1000 for the au pair.