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
Most WWOOFers have a tourist visa. However, it is your responsibility to determine 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" or "volunteer" without a work 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 an employee nor a volunteer. WWOOF France is not responsible for any problems you may encounter with immigration.