You can take a taxi or the bus shuttle. The taxi is 20 euros fixed cost, then there is a baggage supplement per piece.
The bus shuttle is 6 euros and it takes you directly to the SMN train station.
From there, whether you take the bus shuttle or taxi, you'll need to then catch the bus to Siena.
Here are more details (as well as a link to the airport shuttle):
and here for getting to Siena by bus:
The bus is also the best way to get back to Florence after you visit Siena. ENJOY!