Hi suec,
look
here to see a map of Venice and the Aeroporto Marco Polo.. It is better to take the bus (about 30 minutes)from Piazzale Roma (take a shuttle from the ship to Piazzale Roma). Yo can also take a taxi.
More about the bus
here
A taxi costa about 60 Euros from Piazzale Roma to
Airport Airport (about 20 minutes)
Regards
Uwe