If you want star trails, your best bet is to open your aperture really big, use a low ISO, then take a bunch of 30 second exposures while you keep the camera in one place with a tripod (and something like an intervalometer to be able to take 30 second exposures with a certain time in between for say like three hours on its own).
Then, stack them in a program like StarTrails (which is free!). If you use a high ISO and a long exposure for star trails, you'll end up with a noisy photo.
For best results do an exposure around 3 hours long ,, close up the apperture though or it may not be as sharp as it could be , I would also use ISO 100 to reduce the amount of hot pixels.
Use a tripod. Try to aim your camera away from the city and towards where there are less lights(Light pollution will/can ruin your exposure). Use your native ISO (Usually 100 or 200) try to use your sharpest aperture setting (This will depend on your lens. some are f4, some are f16), Use bulb mode + shutter release or a wireless remote, and then calculate what shutter speed you'll need. Good luck!
He said he wanted to get star trails in a comment below you. Your recommendation is more along the lines of just a general starry sky photo. If you want star trails, your best bet is to open your aperture really big, use a low ISO, then take a bunch of 30 second exposures while you keep the camera in one place with a tripod (and something like an intervalometer to be able to take 30 second exposures with a certain time in between for say like three hours on its own).
Then, stack them in a program like StarTrails. If he uses the high ISO and a long exposure for star trails, he'll end up getting a very noisy photo.
As *silentmood mentioned you should use as low ISO as you possible can with the exposure time you want. If you use high ISO there is a greater chance that more noise will appear in the picture.