Venice is a unique city with no other alike in the world. Its public transportation is unique too. One can drive to it, but not in it (with few exceptions, like Pellestrina or Lido, where local residents have a small network of roads). Tourists may walk, take a water taxi or use a Vaporetto (waterbus), of which fares and timetable can be found here: http://actv.avmspa.it/en/content/vaporetto. As there are many types of tickets, check this link carefully to select what works best for you. For a stay longer than a day trip, prices in combination with Venezia Unica Card are cheaper. The card (and waterbus tickets) can be purchased at this link http://www.veneziaunica.it/en/e-commerce/services and includes entrances to museums and churches, depending on the formula chosen, plus a well reduced rate on the public transportation tickets. Always remember to validate your ticket at the machine before boarding the Vaporetto!
Another form of transportation not very well known by tourists is the traghetto: the word means ferry and in Venice is a big gondola that ferries people from one side to the other of the Canal Grande, in areas where there are no bridges to cross. It’s a very cheap and fast way to not only cross the canal, but to be on a gondola indeed! The first traghetto upon arriving is to be found near the train station, by taking a right at the exit in front of the canal and walking alongside of it to traghetto ferrovia. This link shows the other places where they are to be found: http://www.gondolavenezia.it/history.asp?Pag=33. It looks funky and it shows an updated price (it’s now 70 Eurocent for residents and Venezia Unica Card holders and 2 Euro for others), but click on the numbers of the pages and you’ll find brief descriptions on where they are.
Are you driving to Venice? Check your parking options here: http://actv.avmspa.it/en/content/mobility-services and compare prices. It may be cheaper to park in Mestre, in the mainland, and take public transportation into the city, but it may not be so convenient for families with children and/or older people. If you park at Piazzale Roma or Parcheggio Comunale and walk into town, you will be crossing the bridge Ponte della Costituzione, by famous architect Santiago Calatrava: be aware that it can be very slippery when wet.
If arriving by air, check the airport website for your transportation options (bus, taxi, vaporetto -waterbus- or motorboat): http://www.veniceairport.it/en/transport/bus.html.
For visitors with reduced mobility, Venice is a difficult city to enjoy, with its system of narrow ways, bridges and stairs. Check this link for itineraries accessible by wheelchair: http://www.comune.venezia.it/archivio/EN/1381.