Finding a Paranormal Investigator
Once you have determined that the odd occurrences around your house or office are supernatural in origin, the next step should be to hire a reliable ghost investigator. New Yorkers are used to being able to have anything delivered, and these ghost investigators located around the country will be happy to show up at your doorstep.
Paranormal Research Society of North America
The preeminent American ghost investigation agency is the Paranormal Research Society of North America, or PRNSA. Their primary goal is accumulating scientific evidence of paranormal activity, and for the opportunity to use your house as a test case, they would be happy to work for free. They use electronic equipment, not seances or Ouija boards, to detect the presence of spirits. Although they operate out of Pennsylvania, they will gladly come to The Big Apple in pursuit of your ghosts.
Paranormal Activity Investigators
From the Midwest, there are the Paranormal Activity Investigators, who are based in Kansas City. Like the Paranormal Research Society, they do not charge for their services, and their interest is primarily scientific. They have solved ghostly cases in locales as varied as the J. Huston Tavern in Arrow Rock, Missouri; the Historic Pottawattamie County Squirrel Cage Jail in Council Bluffs, Iowa; and the Jack-FM 105.1 Studios in Mission, Kansas.
Grimstone, Inc.
In Michigan, there is Grimstone, Inc., which uses sophisticated video equipment to take the pursuit of the paranormal to the next level. Their digital multiplexers can record up to 16 simultaneous locations, catching any poltergeist who tries to flee through a wall into another part of the house. They also have audio equipment to detect the voices of ghosts that may not be audible to human ears, until recorded. With their Geiger counters and multimeters, they could find out that not only do you have a ghost but the wiring in your house is faulty, which might be helpful to know as well.
Paranormal-NYC
Closer to home, there's Paranormal-NYC, which in addition to dispelling your ghosts will help you conduct research into extra sensory perception, or ESP.
Other Resources
If the ghost hunters have taken apart your house from top to bottom and not found anything, but you are still being kept awake at night by strange apparitions, you may have another resort. According to eHow, if you are a Catholic, you might be able to get a priest to bless your house, hopefully driving out any demons or ghosts. Though this is a task that most priests rarely perform, it's part of the job description, and you should take advantage.