A regular taxi or the bus would drop you off at Piazzale Roma. This is the end of the line for land vehicles in Venice and from here all travel is by boat or on foot. From Piazzale Roma, you can transfer to the Vaporetto, Line 1 or 2, and get off at the San Marco-San Zaccaria terminal. The Vaporetto is the public water transportation system in Venice. The trip from Piazzale Roma to San Marco will go down the Grand Canal, which makes for a nice introduction to Venice.
Another option would be to take the Alilaguna water service directly from the airport. There is a dock at Marco Polo and the Alilaguna Blue Line stops at the same San Marco-San Zaccaria terminal. This options eliminates the transfer at Piazzale Roma and takes about 75 minutes. If the weather is nice this can be a nice option as you cross the lagoon out to Venice.