The ACTV bus company takes you to Piazzale Roma in Venice. Catch the # 5 bus outside arrivals. This is public bus so it does make stops. 
About €8 and takes about 35 minutes.

The ATVO bus company takes you directly to Piazzale Roma, Venice. Here is the schedule. About €8 and takes about 35 minutes.

Hop into a car taxi outside arrivals. This costs about €40 (depending on luggage) and takes about 25 minutes. 

Share Venice-  divide the cost of a water taxi from the airport to your hotel in Venice.  
About €20 per person with 1 suitcase and takes about 35 minutes.

Private water taxi, seating about 10 - 12 people depending on how much luggage you have. From €100 and takes about 30 minutes.
You can catch the water taxi outside near the vaporetto stop.

Take the Alilaguna Vaporetto (public water boat). The vaporetto takes about 90 minutes and tickets are €15 per person and an extra €3 per peice of luggage.

Vaporetti (water buses)
tickets can be bought at several different points:

On board the vaporetto, although they are slightly more expensive.
At the baggage carousel in the aiport
Piazzale Roma (bus depot)
Ferrovia (train station)
Accademia Bridge

75 minutes pass = €7.50
1 day pass = €20
2 days pass = €30
3 days pass = €40
3 days pass for those aged between 6 - 29 years = €28
7 days pass = €60