Galileo Galilei Airport, 1km from Pisa, is the largest in Tuscany (and the one that Ryan Air flies into) and has good connections into Florence. There are several options:
Take the brand new People Mover from the airport into Pisa Centrale Station, then catch one of the frequent trains in an hour to Florence's Stazione Centrale S.M. Novella.
Or take the 70-minute Airport Bus Express direct from the airport to Stazione S.M. Novella. They run roughly ever half hour and you can purchase tickets on line (€12 each way, €6 for ages 2-12).
First departures weekdays from Pisa is at 8.45am, last at 12.20am. First departures from Florence 3.30am, last 9.45pm.
First departure Saturdays and Sundays from Pisa is 9.20am, last at 12.20am. From Florence first departure is 3.30am, last 6.50pm.
Piazzale D'ascanio, 1, 56121 Pisa
Images by: Angelo Romano, Creative Commons License