As you probably know, the earth is constantly spinning. This means that during the course of an evening, the star in the sky will actually move. Photographs tracking the movement of the stars across the sky are called stair trails. There are two ways to get these sorts of photographs. The first one is by using a very long shutter speed, done only in bulb mode. This exposure will need to be quite long, potentially a couple hours so there is always a huge risk that he photo will just blow out from collecting too much light. The alternative and far easier method is known as photo stacking. This is where you take a series of photographs over the course of an evening and then combine them using a lighten blend to create a series of trails. You can combine them easily in free software such as Startrails which will composite all the images together for you. Here is a link to the website where you can download the Startrails software http://startrails.de/
For star trails to look really good, the camera should be in place for a couple hours. This is usually enough to get sufficiently long trails. You can use manual exposure settings with a shutter speed of 30 seconds and an apertures as wide as your lens can do, ideally around 1.8 as we want to be collecting as much light as possible. If your lens cannot go that high, bump up the ISO although be careful to avoid excessive noise. Change the drive mode to continuous and use a remote shutter release cable which will allow you to lock down the shutter button. This means it will continue taking photographs until you come back and switch it off, or until the camera runs out of battery. Long exposure shots chew up a lot of battery so be sure to use a fully charged battery when attempting star trails. Alternatively you can buy a cheap intervalometer off eBay which would let you specify how many shots you want to take. Some Nikon cameras have inbuilt intervalometers so it's worth checking.
10mm 30s F4 ISO 400 Composite of 200 images
For this photo, I set the focus on the hour and then adjusted the exposure settings until I got something that looked alright. Then, I began the exposure and locked the cable release so that the camera would keep taking photos. I stood on top of the wall for the first image and illuminated myself with a torch so that I could be visible. Once the first photo was taken, I simply walked away for a couple hours and let the camera continue taking photos for me. When I came back, I collected the photographs and put them into the Startrails software which blended them together for me.
18mm 60s F4 ISO 100 Combination of 400 Photographs - Queenstown, New Zealand
18mm 30s F3.5 ISO 200 Composite of 100 Photographs- Ocean Pool, Narrabeen, Sydney, Australia
If you've already gone to the effort of gathering so many photos for a startrail photo, you might consider combining them in a video to create a star lapse as this can be a pretty interesting thing to see. Here's a short example of what I mean:
The biggest problem I find is finding a place you can happily wait around long enough for your camera to take enough photos for a startrail. The actual technical side is quite simple, granted you have a remote release/intervalometer. So give it a shot and see what you can do.