Step 1: FIND A GOOD VANTAGE POINT
To capture an unimpeded view of a city or skyline, you generally have two options: choose a high viewpoint, or find some water.
For a high viewpoint, find a building in which you can finagle access to (skyscraper with observation deck, top of a parking garage, friend's apartment, or a building with a green roof.
If your city is near a water source, shoot the city from the opposing bank or get out on a boat - your view is uninterrupted and you have the benefit of reflection for your image too (score!).
Or, achieve both by shooting from bridges, which is a favorite of photographers everywhere. This higher vantage point that bridges generally provide puts you on a more equal plane as the buildings you're shooting, thus eliminating most or all of the distortion skew.
Step 2: CHOOSE A GOOD TIME
Golden hours (hour after sunrise and hour before sunset) are universally good times for photography (that soft golden glow highlighting subjects from the side is treasured in photography). And of course, great skyline photos can be achieved during this time, but, to capture the activity of a city and it's buildings, shoot during blue hour (hour immediately following sunset).
Let's check out the same scene photographed at several different times throughout the day:
SKYLINE IMAGE #1 - Afternoon, hours before sunset
SKYLINE IMAGE #2 - Sunset happening concurrently
Now compare with the image below:
SKYLINE IMAGE #3 - Perfect!
Image #3 was taken 10 minutes after image #2. Just those 10 minutes made all the difference - the sky is darker, there's less ambient light and more artificial lights turned on in the city (and the ones that were already on stand out more because of the lower light).
And, because there was less ambient light at this stage, I was able to expand the shutter speed to 30 seconds. This "stills" the water, creating a more uniform look. There was also a jogger on the river path that doesn't even show up because the shutter was so long.
Step 3: DETERMINE PROPER SETTINGS AND GEAR
The general rule of thumb for skylines, landscapes, and architecture is having everything in focus, thus, a wide depth of field is needed requiring us to put our apertures at anything from f5.6-f22. The largest number isn't always the best. Every lens has a 'sweet spot', play around, find yours (most are 5.6, 8, or 11). Your sweet spot will probably produce the best skyline images, not necessarily f22.
Low = good, of course! Put it at 100 or 200
This is your last adjustment, because the other two are the more important pieces of achieving a sharp, gorgeous skyline image. Because your ISO is low, and your aperture is high, your shutter will likely need to be VERY long. 8 seconds? 30 seconds? A minute? the longer into blue hour you go, the longer your shutter will need to be open.
Pro tip - play around with this further and capture light trails from cars, boats, or or other moving lights to further add interesting visuals to your skyline image.
If you're hand-holding your shots, without the use of a tripod, try at least grounding it on a pillar, a railing, a door jam....anything. Then, you can get away with less desirable settings in order to at least get some decend shots; try ISO 800, f4, and as high as the shutter will go. With a wide angle or a ligiht-weight lens you could maybe get away with 1/15 (and throw out 9 out of 10 blurry shots). With a zoom, you're just out of luck trying to hand hold that at night and get a clear shot without ratcheting up the ISO to ungodly grainy levels.
Additional gear to bring
Tripod = a must for long exposures
Shutter release = avoid the small jostle you make to your camera when hitting the shutter on the camera body. Pick up a generic shutter release for your camera from B&H for around $10.
Filters = if you have ND filters or something similar bring them! They'll help you shoot more effectively when there's more ambient light.
Flask / joint / coffee / favorite bag of chips = keep yourself happy while waiting impatiently for several minutes every time you take one image.
That's it! You're good to go! Dig out that tripod that never gets any use and scamper up some bridges and parking garages and have some fun!