I have the Find My iPhone app. after locating the area how can I get the exact address?
To add to what James said, also consider screenshotting any locations that show up for a missing/stolen phone. It's only a matter of time before the phone goes "off the grid" and can't be found, and there has been some success in getting law enforcement to act upon this sort of information.
Jan's answer is true, but Find My iPhone also provides you wish accuracy information in the form of a blue circle around the location; the larger the circle, the more inaccurate it is. If there is no blue circle, it's confident to within about 5 metres of the location. At that point, you can open up google maps and cross-reference - place the street view man down and you should be able to see an address on the front of the house/building if it's an urban area, or the street view guy itself should list an address for rural places.
I did this; and got an accurate address to cross-reference against an employee database, and it matched perfectly.
Location services are always estimates and it's accuracy depends on multiple factors, mostly the method used to pinpoint the location.
For example tracking via cell towers will usually result in a large overlapping area where the phone is located. You may be looking at an area with a radius of 100-1000m. Tracking via GPS on the other hand should be rather accurate ~ on the outside maybe as accurate as a few meters.
Usually it will also be more accurate in a populated area, a city than in a remote area.
Because of that Find my iPhone will never give you a definite address but rather a pinpoint of it's most likely location.