it is not so far from S. Lucia, the Venice rail station, to Stazione Marittima, the Venice cruise terminal. If you have not many pieces of luggage, you can walk from S. Lucia over a white bridge on the other side of the Canal Grande. Than go right side to the next (smaller)bridge; cross it to Piazzale Roma. There take a taxi (only some Euros) to the cruise center.
Or you take the line 1 vaporetto from the rail station to Piazzale Roma, than take a taxi. Maybe your Cruise Line will have a shuttle bus.
for some more information.
you will find a map with the rail station and the cruise center.
Last but not least here
will find find a nice map of Venice.