Set your camera and lens to Manual
Set your ISO to 800/1600 (Or more depending on your camera)
Set your Aperture (f-stop) to f-2.8 or the lowest you can)
Set your shutter speed, 20 sec and test from there
Always remember your tripod!
Zoom out as much you can, and set your focus to ∞ (infinity)
If you can, use a remote control for your shutter, if not use self timer.