From the cruise ship terminal walk to the train station. There you will find the Vaporetto. If you want to walk there (and see the sights along the way) walk through the train station and head left along the main street.
If you generally just follow other people you will eventually get to St Marks Square. Walking at fast (New York) pace it will take about 1/2 hour, a slow walk in an hour.
There will be a few places where you can take a few different short walkways, but they generally lead to the same place - just make sure you are in a flow of fast walking people - then you know you are on the main walkway through the city.