After a few days in Venice, you may wish to take off and see other parts of Italy. I personally felt three days was perfect and shot down to Florence for a few days.
The Venice train station is both an easy walk from most places in the city and a stop on the Grand Canal vaporetto. It has an expansive staircase, where we found ourselves sitting, basking in the sunlight, rather than waiting indoors.
To buy a ticket, it is best to go first to the Information Booth where you can request a schedule. Explain where you would like to go and that you would like to see a schedule of trains. I did that in Italian, thanks to my "Speak Italian in 7 Days" book. The info booth worker will circle a few possible trains and perhaps explain the difference between one and another. Take the horario up to the ticket counter and use it to help you buy your tickets. After all, while many of us can express that we want to buy a train ticket, the vocabulary to specify which kind is quite challenging.
You have purchased your ticket, but now you have to get it validated.
The price differential between first class and second-class is small, but the luxury is apparent. On our trip to Florence, we took first-class. We were the only people in our compartment and we closed the shade to the hallway. Then we relaxed in our comfortable chairs, looking out the large windows. There was a little table for eating and lots of luggage storage space. It was almost bearable when the train had to stop for an hour at a small town, which was not on the schedule.
On the way back, we purchased first-class but we didn't get first-class seats. The train was already packed when it pulled into Florence. We boarded in first class and walked down the entire way without finding seats. When we did try to sit down, we were told that the seats were reserved. Note to self - reserve seats next time! There was only one first-class car and we walked through quite a few second-class cars before we found a seat. Technical, first-class passengers are not supposed to sit in second-class, but it was our only option. Many people had put their suitcases in the aisles, meaning that we were unable to pass without lifting up our heavy rolling suitcases to get by. I was also carrying two duffle bags and a purse on top of this. I am not ashamed to say that when we could not find a seat, I actually started to cry. Luckily, a nice Italian man came to my rescue and found us some empty seats at last.
The second-class compartments are smaller, with three seats abreast instead of two. There is no under-seat luggage storage, only a smaller area overhead. Also, there are many children in second-class and it is their greatest joy to run screaming down the aisle, tripping over suitcases all the while.
In summary, I highly suggest buying first class, reserving your seats, and don't travel during Easter Week. I am marking this entry as 'athletic' as it required some athleticism to lift my suitcase numerous times over other people's suitcases. Plus running down the aisle looking for empty seats before the train departed counted as my cardio for the day!