Ciao Rosalba,
Since you'll already by arriving by train into Florence, the easiest and simplest choice would be to rent the car in Florence and then drive down to the Val d'Orcia area. Getting there with public transport is possible but it would take longer - you can take the train from Florence to Buonconvento (south of Siena - requires train change in Empoli) and then a bus to wherever you are located. If you aren't staying in a town, buses drop you off along the main roads they travel and it might require a long walk to your accommodation. So the logistics are a bit more complicated and that is why it would be easier to just get the car in Florence - although Empoli or Siena are other good possibilities if you want to rent closer to your destination as both of these are easy to get to by train.