You certainly won't be needing a car if you're only planning to visit Venice: there's water where the streets should be. There are roads and motor vehicles on the Lido but it's such a small island, and parking is a nightmare, that you'll undoubtedly find a car is more hindrance than help. The only instance in which a vehicle is vital is if you're planning to explore the mainland Veneto region. For information on car hire, see my Venice car hire page.
If you arrive in Venice by car, you will have to leave your car at one of the car parks on the mainland, at Piazzale Roma or on Tronchetto island. Many hotels have special deals with a car park: you should ask your hotel before choosing one at random.