The regional trains heading to Chiusi all originate at SMN, so show up a half hour early and buy your ticket, then validate your ticket and board the train as soon as it is spotted on the platform. You'll be among the first on board and will have your pick of seats.
PS There is liable to be long lines at the ticket windows, Florence being a tourist town, so use the ticket machines with a credit card. Second thought, since it is your first time in Italy, maybe buy it the day before to practice. Just remember, the Italian name for the city is Firenze. Tickets bought this way are "open" tickets, so you must validate them in the stamping machines on the platform just before boarding the train. Failure to do this will get you fined.