Bergamo's city bus network (ATB) is the main way to get around without a car. Tickets must be purchased before boarding — from the ATB office at Largo Porta Nuova, the train station, automatic machines, or the ticket window at the bus terminal (Stazione Autolinee). Validate your ticket on the machine inside the bus as soon as you board.
Bus 1 runs from the train station past Porta Nuova up to the funicular that climbs to Città Alta, the walled upper town. A tourist day pass covers multiple zones including both funiculars — one up to Città Alta and a second continuing to San Vigilio — and is worth it for a day of exploring. The airport bus (Bus 1C) connects Orio al Serio airport to central Bergamo in around 15 minutes and departs every half hour throughout the day.
Current fares and timetables are on the ATB website at atb.bergamo.it.