Find the Scene
The first thing you need to do is find a scene that has as little light pollution as possible whether its from city lights from a distance if shooting out in the sticks or even nearby street lamps if deciding to shoot in the city itself, these will be greatly exaggerated as the shutter will be open for longer than 10 seconds and has the potential of ruining the shot.
You want to set your camera to Manual shooting mode, manual focus on infinity and open your aperture to the widest setting (lowest f number), sometimes I increase the f-number a couple of clicks as some lenses dont perform best at its widest and this will also bring out more of the colours in the stars. You will also want to turn off the noise reduction in the camera, otherwise you will be sitting and waiting for the noise reduction to take place.
Once you have found your spot to shoot, composition will be hard to achieve at first as its most likely very dark and although you will have a general idea of the direction you would like to shoot, it will be hard to tell if its align and perfectly composed the way you like, so bump up the ISO to its maximum setting, and take a series of test shots of about 1s shifting the camera to find the desired composition. The photos will be unusable and really noisy but you can see the general details in how you would like to compose the shot.
Now that you have the composition, drop your ISO to about 400, and set the shutter speed according to the rule of 600.The rule of 600 states that the longest shutter speed you can set to avoid capturing the movements of stars is 600 divided by the focal length of the lens. So if you are shooting at 20mm, 600/20 = 30s. Remember that for cropped sensor cameras you will need to multiply the focal length by 1.5 for nikon users and 1.6 for canon. So if i was to shoot on my D7000 with my 18-50mm lens at 18mm, the longest shutter speed i can use is 600/(18×1.5) = 600/27 = 22s.
Taking the Shot
Now you have the aperture, iso and the shutter speed set, you are ready to take the photos. A lot of the times i would increase the iso in steps up to 3200 to see if i can bring out more of the ”hidden” stars, and maybe the details in the milky way, but try and choose the photo you are happy with that has the lowest iso setting as it will have the least amount of noise.