Try the Trenitalia website or trainline.eu (which includes both Trenitalia trains and Italo trains - two competing rail companies, Trenitalia being larger and the national railroad).
http://www.trenitalia.com/tcom-en
https://www.trainline.eu
For the Treitalia site, you must use Italian spellings for cities (Venezia not Venice).
Look up schedules. Note that there is no point or need to buy regional train tickets ahead of time - those trains aren't reserved and can't sell out, plus they cost the same to book today as they would 20 minutes before departure. If you aren't sure of the time you might take a regional train, wait to book your tickets til you get to Venice.
The faster express trains (reserved seats) do tend to be cheaper to buy ahead of time, as cheaper seats available at lower prices sell out the closer you get to departure. So if you can be fixed in your plans, you can usually save money booking those ahead of time. You can either book now and download/print out your tickets or show them on your phone or tablet (or both).