There are two ways you can become an au pair:

Your first option is to join an agency. This can be costly (as some agencies charge au pairs) and doesn't necesarily guarantee success with your family. Having an agency as your liason will offer you some peace of mind in your initial meeting and if something does go wrong. When going through an agency, they will help you set up your contract and assist in obtaining a visa. Be aware that different agencies will have different aspects contained in the contract so it will be advantageous to find an agency with a contract to your liking before committing.  Also be aware that you should technically not be paying the agency a fee.  Its the family who pay the agency for finding you so you should not be paying anything.

The second and least expensive way to go about becoming an au pair is to do all the leg work yourself. You will have to find a family, set up phone interviews and eventually negotiate your contract. The family should also register with the DDTEFP a french government entity responsible for work and professional training regulation and authorization.  Be aware that you will be doing this yourself so again you will have no guarantess with the family, so do as many checks with the family as possible ie. speak to the previous au pair etc. (check the "Negociating your contract section")

Whichever route you decide to take, you should be aware that there have been au pair laws established in France. Unfortunately the laws have not been updated since 1969 when they were initially established. It is vital that you be aware of what is and is not required by law and subsequently enforce them while working for your family. This will ensure a good and professional working environment.