hi, first of all keep ISO as low as possible for your purpose, high ISO means a lot of noise (and a lot of in camera noise reduction that lowers image details). In my little experience you just should lock focus on infinity and set manually the time value that looks nicer to you, with aperture stuck at maximum (to reduce time value) or 1 or 2 steps from it to reduce lens problems at borders (i think it's called "coma"). Also white balance can be set accordingly with your taste. Another variable is dark frame subtraction, you can do it by hand on a computer with only a few dark frames taken by hand or let the camera do it after every shoot (best results but you double the minimum delay between shots). Also use a very stable tripod with a remote shutter or set the auto-shutter time to delay your on camera shutter pression by a few seconds to reduce vibrations. Only when you've found your right set up start the time lapse.
However you can find a lot of info on night shooting on the net, much more than here.
