I am going to Venice in the next weeks and specially during the night I would like to have these 2 kind of results if possible with my camera.
Can you recommend me the settings i would need to use for these kind of pictures at nigh?
http://farm9.staticflickr.com/8234/8518268990_7f71780985_h.jpg
You need a tripod, set your camera for long exposure, as low an ISO you can set.
You need to experiment on the exposure time to get the proper result.
Set focus to infinite.
Set a 2 second timer and mirror lockup to prevent "shake".
Shoot RAW and post process.
I suggest finding sites during the day and come back a night when there are less people in the streets.
Good luck.
I suspect the first picture you show wasn't really taken at night. It was probably taken at dusk after the lights came on but before the sky illumination went away completely. This helps fill in areas that would otherwise be very dark. Note that the ripples of the water can be clearly seen, so this was not a typical "long" nighttime exposure. It was probably 1/2 second or less. That still means a tripod is a good idea though.
The second picture seems to have been taken deeper into night, and clearly a long exposure was used, as can be seen from the texture of the water surface. This is mostly a matter of using a long (several seconds up to maybe 30 seconds) exposure.
The tricky part of night pictures with artificial lights in the picture is that the contrast ratio is very high. The lights themselves will be blown out in order to get any reasonable brightness resolution on the ordinary objects illuminated by those lights. The very bright spots of the lights themselves can cause various artifacts. A low-dispersion lens will help, as will a wide aperture. The diffraction effects of a small aperture that are not visible for normal scenes can stick out when the bright lights are in the picture themselves. Fortunately for such wide scenics, everything is far enough away that low depth of field from a wide aperture is not a problem.
Unless you have a high end camera, this kind of lighting is going to put you in to >1 second exposures. You'll want a tripod for that. In general, you should be able to focus pretty easily by hand, but auto focus likely isn't going to be much help. You might have luck with contrast based AF, but it depends on how good your sensor is.
You'll have to experiment with shutter times, but most DSLRs can go up to 30 second exposures without issue and at those kinds of light levels, you should be fine well under 30 seconds even at ISO 100. Keep your ISO as high as you can to minimize noise, but be aware that any movement during the exposure will result in blurs where that movement occurred (assuming it is lit). Particularly in a city, this may end up resulting in wanting to use shorter exposures with a faster ISO simply so you can avoid unwanted movement in the scene.
Certainly shoot RAW to give yourself some room to adjust after the fact and if your camera has it, I'd also suggest using long exposure noise reduction to help reduce accumulated noise from the long exposure.
If you have the budget for it (or have one already) using the fastest lens you can will also help as it will let additional light in to the camera. If you are shooting on APS-C cameras you can get some pretty decent and cheap fast prime lenses for a couple hundred bucks.
Besides the obvious "buy a tripod" advice which is the number one point for any night scenes that involve static objects (buildings, landscapes), there might have also been some HDR editing applied to at least one of the photos - specifically the Paris one. (Something about the colours strikes me as the Photoshop HDR tool.)
An HDR image is created by having a number of over and underexposed images (minimum two, depending on the scene also more) to cover the the dynamic range of the scene. This can help with nightime images that have very dark and bright "spots" and lead to better/more impressive results than a single long exposure.
Having said that - a long exposure on a tripod would be the starting point - ideally with an ISO low enough to offer good dynamic range and low noise, the exposure is then selected to match and the aperture is possibly not too wide to give sufficient depth of field.
