Zurich's public transport — trams, trolleybuses, suburban trains, and lake boats — is run under the ZVV umbrella and it is excellent. Trams run roughly every six minutes during the day and cover everywhere a visitor is likely to want to go. Buy a day ticket (Tageskarte) from the machine at any stop before you board; tickets are not sold on board. The ZürichCARD covers unlimited transport plus museum discounts and is often a good deal for tourists.
The old town is small and flat, and walking from the main station to the lake along Bahnhofstrasse takes twenty minutes. In summer the city runs a free bike share — pick one up at the station with an ID deposit. Taxis are plentiful but extremely expensive; rideshare apps (Uber, Bolt) are usually cheaper.