The transportation system in Zurich is complicated, with multiple zones and many pricing options for time of day and travel duration.
I think the simplest option is to buy a Zürich Card which is good for travel for 72-hour on all the various transit: tram, bus, train, boat and cable car. Information available on the ZVV website.
Rick Steves recommends the SBB app for buying tickets in Switzerland, including ZVV tickets. It has the advantage of holding your credit card information and being able to buy ZVV tickets using just the app — tickets are held as QR codes. Download the iPhone or android app.
