The technique you are looking for is called "Slow Sync Flash". On your Canon camera, you enable it by turning you mode dial to "Av" (Aperture priority). In this mode, the flash will fire to expose the foreground, but the shutter will still remain open for a long exposure of the background, just like when you shoot without flash.
Beware, that in this mode, you have to expect subject movement during exposure. The flash freezes the motion, but depending on lighting conditions, you may still get "ghost" images.
If you want the backgound to be visible, but darker than your foreground, you have to dial in some negative exposure compensation (by turning you wheel while holding down the "Av" button).
ISO, speed, and aperture really depend on your specific situation, there can be no general rule. Your ISO should be as low as you can get away with, aperture has to be set to achive the depth of field you want, shutter speed results from the exposure you want to achive.