For your camera...
Put the camera in manual mode. Then, just rotate the "main command dial" (the knurled dial facing you on the upper RH side of the back of the camera... NOT the one on the front below the shutter button, which controls aperture). Note the numbers changing on the lower LH side of your viewfinder as you rotate the dial. That's the shutter speed. If the number is increasing and there is no "seconds" symbol (") to the right of the number, that means the shutter speed setting is getting faster (i.e. 500 = 1/500th of a sec., 1000 = 1/1000th of a sec., etc.). Rotate the dial to the right for faster shutter speeds, left to slow down shutter speed. Keep turning to the left until you see the " symbol after the shutter speed number. When you see the " sign after the number, that no longer means 1/X of a second, it means number of seconds. So, 6" means 6 seconds rather than 1/6 second. Experiment with 1 - 8 seconds or so at first. Depending on your focal length and aperture, you may need more, but you'll generally get a good night shot within that range if there is any light at all in the scene.
You do the same thing when in Shutter Priority mode, except that in Shutter Priority, you control the shutter speed and the camera controls aperture. In Manual mode, you control BOTH shutter speed and aperture... and ISO as well, if you have "auto ISO" turned off.