If you’re going to Siena without a car, I recommend you read this article on getting to and visiting Siena without a car which has information on the various ways to get there. If you do drive to Siena, then you’ll certainly find useful information on where to park in this article on Parking in Siena.
A day in Siena is certainly not enough to experience the city in all of its many sides, nor to visit all of its treasures. In a day, however, with a bit of organization, you’ll certainly be able to see the main attractions of Siena and to appreciate the charm and atmosphere of this lovely medieval town, probably leaving with the desire to return as soon as possible. If you have the time in Tuscany, do return for a second day!