Info 2 : First, set your ISO to 3200. Next, set the EXPOSURE METERING option ( page 79 of your manual ) to SPOT. Then set AUTOFOCUS to MANUAL ( page 55 ) on both your D3100 and on your lens. That is because it will be doubtful that there will be enough light for the camera to autofocus properly.
Info 3 : Next, set the Command Dial to P for Programmed mode. Then zoom in on a lighted area in the night scene you are looking at that you want to expose for, placing the center of the viewfinder on this lighted area. Then press the shutter, and note what recommended exposure settings ( aperture and shutter speed ) are displayed.
Info 4 : Make a note of these two settings inside your head, and then switch the Command Dial to MANUAL mode, and manually set your shutter speed and aperture to match those recommended settings that you noted earlier. Then zoom your lens to compose the photo the way that you want to record it, and then set your focus manually.
Info 5 : Take a photo at those settings, and then review it and see how it came out. This is only a starting point, and you may well have to adjust either the aperture or shutter speed, to get a better looking exposure. And be careful that the shutter speed is not set too slow, or you may not be able to handhold the camera and still get a sharp photo. Assuming that you are using a lens with the VR optical image stabilization, I would recommend not going any slower than 1/15th second, or at the very most 1/8th second if you have already practiced holding your camera super steady.
source: FB Nikon