Hi hyori,
To shoot portraiture at night, if you don't mind the background, just turn on the flash and shoot. If you want to include the background, you'll need to use a tripod plus show sync flash. The subject must remain still throughout from the moment the shutter is released till the shot is taken. If you're using A mode, make sure the subject is within the flash range for the selected aperture, ISO and focal length (refer to your camera's manual if using the internal flash, or the manual for the external flash).