If you are a citizen of the European Union
You do not need a visa.
If you are NOT a citizen of the European Union
You can perform WWOOFing activities as a tourist, usually up to 90 days. Depending on your country of citizenship you may be required to obtain a Schengen tourist visa (see list here). It is your responsibility to determine and obtain the appropriate visa for your stay. Neither WWOOF France nor hosts can help you obtain a visa. It may be best to secure your visa before becoming a member of WWOOF France.
Keep in mind that WWOOF is not paid work nor a paid volunteer program. The WWOOF is an educational experience, and the WWOOFers are the guests of their hosts. The European Union and France have strict labour and immigration laws prohibiting foreigners (non-EU) to "work" without a proper visa. If you say you are coming to "do volunteer work on a farm" and you do not have a work visa, immigration officers will likely not let you in.
Also, note that when you enter the European Union, you cannot enter as a "WWOOFer" since this term could be misunderstood by immigration officials. So be aware of this important distinction: as a WWOOFer, you are a tourist, not a worker nor a volunteer. WWOOF France is not responsible for any problems you may encounter with immigration.