This really does depend on when the flight is, how much luggage you expect to have, whether you are willing to change airlines en-route, and whether you are looking for direct flights.
Take a look at the airport website (http://www.europe-airports.com/italy...?code=VCE) for
ideas of route oprions AA fly Venice to Madrid & onto the USA, whilst UA fly via Frankfurt
If you are happy to change in Europe, then both Amsterdam (Schiphol) and London (Heathrow and Gatwick) have several flights a day from Venice, and then on to Houston. This includes budget airlines such as EasyJet, RyanAir or Transavia, KLM, and BA (If they are not on strike).
So look for a few routes, then shop around on line options.
Alan & Katrina