china,
As the others have mentioned, travel by Bus from Florence to Siena is more convenient, as passengers are dropped at Piazza Gramsci in the center of town, rather than the rail station which is at the bottom of the hill. The Corse Rapide (express) buses only take about an hour each way, so still very fast.
As mentioned, the Bus station is very close to Firenze SMN rail station. If you're going to be in Florence for a few days prior to your trip to Siena, I'd suggest buying your Bus tickets the day prior to travel, and then validating them prior to boarding the bus. No need to buy those in advance (IMO).
For "places to see" in Siena, some of the usual favourites are the beautiful Duomo and of course Piazza del Campo, where the Palio is held each year. Note that there's a dress code in the Duomo, so no bare shoulders or short pants! The guidebook has lots of information on other sights, as well as good restaurants, etc.