If you're checking in at the main Marittima cruise basin, you can take the inexpensive (and barrier-free) People Mover automated tram from Piazzale Roma to the cruise terminals. After you've checked in, you'll board your ship or--if it's a larger vessel--you'll be bused to the ship's pier on the Italian mainland.
If you're staying in Venice before or after your cruise, we strongly urge you to read Venice Cruise Terminal Hotels or 17 Hotels Closest to Cruise Ships before choosing a hotel location.
Do not assume that your cruise-travel agent knows Venice. We often get panicky e-mails from cruisers who have been booked into hotels that are a long distance on foot or a painfully expensive water-taxi ride) from the cruise piers. It isn't unusual for passengers to be booked into a popular chain hotel that is nearly 28 km or 18 miles from Venice by train.

More transportation advice:

Arriving in Venice
How to reach the city by plane, train, car, or ship, with links to articles on airport transportation and related topics.

Getting Around Venice
An introduction to Venice's water transportation system, with links to articles on vaporetti, gondolas, traghetto ferries across the Grand Canal, etc.

Accessible Venice
If you use a wheelchair, can't walk long distances, or have trouble with steps (e.g., on bridges), read this article before you arrive in Venice. The article includes links to special maps and other local resources in English.