Long exposure is a broad term and actually has a series of techniques that come under it. Long exposure is basically anything with a long shutter speed. You can do this using either Tv or M modes on your camera and changing the scroll wheel to set a long shutter speed. Long exposure usually requires a tripod as you generally can't hold it still for more than 1/60 seconds when shooting handheld. Having said that, you don't always need to use a tripod, but it usually helps. When doing long exposure stuff, it's good to use either a remote of if you don't have a remote, use the 2 second delay option under drive modes.
Long exposure is a very useful technique when doing night photography along with a tripod. This is because the camera needs some time to collect enough light properly expose the image. The camera will want to use a high ISO to try and keep the shutter speed short. However, if you have a tripod, then a slow shutter speed is not an issue and as such, unless you have moving objects, ISO 100 should be used. It's also good to use a low aperture, maybe around f8.0 or higher like f11 you're taking a photo of nightscape because this will ensure sharp photos and a large depth of field. This is because most lenses have a sharpness sweet spot around the middle apertures.
If you don't have a tripod, then you'll try to shoot with the highest aperture and ISO possible without making overly grainy photos. Here are some example night time shots using a tripod. These are long exposures as they need several seconds to collect the light. A nice thing about long exposure is that if there are people moving across your frame, they will mostly likely not show up in the final photo. This is because an objects prevalence in a photograph is dependent on the time it spends in that part of the frame and how bright the object is. As such, dark objects need to be exposed for a long period of time to be visible while bright light can be exposed fora fraction of a second before imprinting themselves in the photograph.
Night photography is often about timing. It's best to do your cityscapes during blue hour which is just before sunrise or just after sunset. This is when you have that beautifully rich blue sky which looks excellent in your photos. If you wait until too late in the evening, then the sky will be black and your photo will likely not look as good. The following is an example shot showing the same location with roughly the same settings except one is taken during blue hour and the other is taken later at night. SO this way you can see the difference that it makes.
To give you another example of how timing and weather is important in photography, take a look at the following photos of the Sydney Harbour Bridge taking at different times of the day and different weather conditions.
Sometimes you have to shoot scenes through glass. The problem with shooting through glass is that you have reflections that show up on the glass. To avoid this from happening, you should get the camera really close to the glass and but also hold a piece of black clothing such as a black jumper over the camera to block out the reflections on the glass. The following is an image that I took using this technique.
10mm 30s F8 ISO 100 - 365 George St, Sydney, Australia
Another thing to keep in mind is that if you have a lot of street lights in your shot, you can use a small aperture to give your photos a start like effect. The smaller the aperture, the more pronounced the effect. You can look at the photo below to see what I'm talking about.
16mm 72s F16 ISO 100 - New Brighton Pier, Christchurch, New Zealand
Night photos are a great way to introduce yourself to long exposure photography. Just make sure you grab yourself a tripod and watch the weather and the time to help you get the best shots.