To photograph star trails, you need to be able to set your camera on shutter priority, on the "B" (Bulb) setting and you also need to have a remote shutter release, which will allow you to keep the shutter open for extended periods, from 2 - 6 hours. And, not forgetting of course, the trusty old tripod.
You can set your lens as you normally would for landscapes, or on its widest aperture and focus set to the infinity mark on the lens. Press and lock the button on the remote shutter release and take a note of the time.
To get reasonable star trails, you will need to leave your shutter open for at least 2 hours, but the longer the better. Oh, and for these long exposures, take along a couple of extra sets of batteries, you might need them.
A good viewpoint is when you have your horizon line just showing above the bottom of your image and the sky, not pitch black, so as to show a line of distant trees or the rooftops of buildings. Or, for added drama, set yourself up in a cemetery and use the gravestones and/or a tree as a backdrop and whilst the shutter is open, shine a torch briefly over the tree or gravestones to create that eerie effect, or you can use your imagination to create whatever effect you want.
For circular trails you will want to be facing South, or for diagonal trails point your camera East or West. If unsure, it pays to take along a compass.