Parking in Venice is not easy and unfortunately most of the options available in the tourist areas are chargeable.
Once you are in the vicinity of Venice, cross the Ponte della Libertà' (3850m) and you will easily reach Piazzale Roma on the island of Venice. In Piazzale Roma the most popular parking option is the municipal car park or garage.
The other island parking option is on Isola Nova or Tronchetto, the Tronchetto Parking.
Parking on the "mainland", in Venezia Mestre, is the cheapest and least problematic solution. Here are some solutions.