i am not professional photographer but i will tell you what I know. you can change the exposure time (shutter speed) in manual mode or the S-mode. long exposure is good for night scene specially when you want to use low iso speed. i try many shots to find the best shutter speed. Also long exposure is nice for many scene like waterscapes and sunsets.. its also important to manage the light which enters the camera by making the aperture narrower (high f value), decrease the exposure, and decrease the iso speed. For long exposure you need also a tripod.
You can simply set the camera to the manual mode and play with the values ... Good luck :))