As others have said...night shots come in two primary flavors - shots in which you are trying to photograph people or movement or action at night...or shots in which you are trying to photograph scenics, landscapes, buildings, or other still scenes at night.
With the first example, you need to try to 'freeze' the action - your subject could move, so long exposures won't work, so the basic idea is to go with a fairly wide-open aperture (low F number)...usually easiest to attain in A (Av) priority mode, and then raising the ISO until you have a shutter speed of at least 1/50 to 1/100 or so. You might get by with slower shutters down to 1/15 or 1/30, if you tell your subject to try to remain still.
With the second example, the afformentioned tripod must come out (or at least find a nice level surface to put your camera down on), and then you use nice, long shutter speeds as needed to take a longer exposure and let the camera suck up all the light it needs. Night shots will vary widely on how much light is needed - a well-lit Disney pavilion at night for example might only need 5 seconds, while a quiet boat dock on a dark lake might need anywhere from 30 seconds to several minutes. An easy way again is to use Av priority mode - set it where you want/need it for depth of field, then let the camera decide when it has received enough light to get the right exposure. Manual mode is best once you get the hang of it, but Av priority can help you start to understand how long of a shutter to use for different lighting and different aperture settings. As mentioned above, a remote shutter release is suggested so you don't touch the camera...or use the self-timer so you can press the shutter and get your hands off the camera before it triggers.
