Let me start with stating the obvious: you should not drive to Venice. It is best to take one of many forms of public transportation available in Italy.
But if you must, you will find that parking for Venice is quite expensive at around €25 euros per 24 hours. There are plenty of parking options at that rate around Venezia-Mestre which is the main train station on the main-land that connects to Venice. You can park your car in one of these parking garages, walk down to the train station, hop on a train and you will be in Venice in no time.
However, if you find that rate excessive, there are more affordable parking options available away from the train station. One such option is Garage Lazzaro which is just a few bus stops away from the terminal bus station in Venice. At at €5 euros a day, it is a steal!
We had no issues finding the garage using Google maps on my phone. We had to prepay €20 euros (in cash) for the four days we planned on staying there. We were handed a parking ticket and shown an area in the gated lot next to the building where we could park our car. The lot had plenty of empty spaces and felt secure. We did not have to leave our keys with them which was a plus.
After locking up our rental, we grabbed our carry-ons and headed over to the bus stop across the street. Be warned, you must climb up and down the stairs in order to use the pedestrian over pass to get across the street (there was an elevator to access the pedestrian bridge, however it looked like it has been out of commission for many years). Once across the street, we jumped on the number 9 bus using the tickets we purchased on our phones. We were in Venice in about 15 minutes.
We returned three days later, showed them our prepaid ticket and drove off without any issues.
You can find out more about the app which can also be used to purchase water taxi tickets on the official public transportation website for Venice http://actv.avmspa.it/en/content/avm-venezia-official-app.