It's easiest to use Kayak: http://www.kayak.com/flights.
Put in Houston as your starting point, Venice as your destination, and see what comes up. You can then use the filters on the left to select one-stop flights, choose how long a connection you will tolerate, select connecting airports or airlines, etc - very handy.
You'll see that you can change in JFK or PHL (I think sometimes there's a nonstop from ATL as well), but you may prefer to fly nonstop from IAH to Europe, then change there to VCE. This will probably be shorter overall, and you have fewer problems in the event of a delay. From PHL and JFK, there's only one flight to VCE a day, but from, say, Paris or Frankfurt, there's several a day.
However, if you're returning from VCE to the US, the flights that connect in Europe often leave a very early hours. Getting from Venice to the airport early is expensive, time-consuming, or both. So you may prefer to use the PHL or JFK flight on the way back, as they leave somewhat later.