Bus 7/7x run regularly to Fiesole, a journey of around 20 minutes.
The buses now leave from near Piazza San Pietro, with the stop sited along Via Giorgio La Pira (the side furthest from the church). See photo.
You can buy tickets from the newsagents booth in the centre of the piazza (see photo). If there are two of you a 4-journey ticket costs 4.70 euro. Make sure you validate it twice (numbers 1 and 2) on the way out and twice (3 and 4) on the way back. There are on the spot fines for non-validation and inspectrors do check (they caught someone on my bus).
You can also buy tickets from the driver on this route, but at 2 euro for a single journey. So buying from the newsagent ( a very helpful man with good English on duty when I visited)is a better option.
The bus stops in the main square at Fiesole, and goes up a rather exciting hairpin road to get there...excellent views!
Bus timetables and info in English on www.ataf.net
Getting to Fiesole from Florence using public transportation is dead easy. Bus No. 7 leaves from Piazza di San Marco every 15 minutes (every 30 min after 10:00 pm). Tickets cost 1.20 Euro and you can buy them from the ticket machines located on the piazza (don't forget to validate your ticket once you get on the bus). It's probably a good idea to buy your return ticket at the same time since there are no ticket machines in Fiesole (or at least we couldn't find one). It takes about 20 minutes to get to Fiesole by bus, which is the very last stop on the route, right next to the village's main piazza. Buses run on a similar schedule to get back to Florence, i.e. every 15 minutes (every 30 min after 9:30 pm), and the last one leaves at 1:00 am. Enjoy the ride!
ATAF Buses leave Florence at regular intervals for the trip up into the hillside town of Fiesole 8kms away. Buses leave from Santa Maria Novella outside the southeast entrance/exit of the station below Piazza Adua.
Take the No 7 bus and stay on until the end. Stops before Fiesole main square are Regresso and F.G. Angelico 03. Buses leave every twenty minutes and the trip takes 20 to 30 minutes depending on traffic conditions.