From the City's website:
To Become a Block Watch...
... all you have to do is talk with your neighbors and see if they are interested in working together. Then call the New Haven Police Department Neighborhood Services at 946-6299 and arrange for a representative to come out and meet with your group. Most meetings are held in evening to accommodate people's busy schedules, and they are usually in someone's home to make attendance easier. If you live in an apartment building and no one's home is big enough for a meeting you can get together in your community room or ask people to bring their own chairs to the laundry room, or even meet in a large hallway!
There is more info under Police Department at http://www.cityofnewhaven.com/Police/CrimePreventionSafety.asp#BlockWatch