There's different ways to do it, and it depends on what you're shooting. If you're shooting people you pretty much need flash or to put them in whatever available light you have. If you’re shooting things that are immobile then for best results use a tripod on a long(er) shutter speed in manual mode. If you’re doing handheld at night, then things get a bit trickier and don’t expect a good yield percentage. That said, when you do get a good crisp night shot handheld, they can be awesome.
I prefer shooting on full manual. The problem with auto and semi-auto settings is that if you leave the exposure up to the camera it will try to equalize things to a middle grey. It doesn’t know that it’s supposed to be dark so it will overexpose in many situations. You can simply turn down the exposure compensation if you’re more comfortable in a semi-auto like Av. Your camera might also have a night mode that aims for a darker exposure, I can’t really speak to it since I don’t use those settings.
Here’s what I do. I grab my fastest lens, usually a 50mm prime or something. Keep in mind that the longer lens you use the faster shutter speed you need to get a sharp photo. I turn to Manual, and open the aperture almost all the way (I know that my 50/1.4 is sharper at f/2, and it gives me a bit more depth of field). I then move the shutter as slow as I think I can handhold, with a 50mm I’ll usually go to 1/40, knowing that I’ll miss a fair amount. Then I turn up my ISO until I get the exposure that I like; with a T3i and a kit lens you’re going to be well into the ‘high ISO’ zone. When I take shots I get a nice stable position with my arms and lean against something if I can. I shoot in bursts, at least three, so there’s no camera movement from pushing the button. I chimp constantly (check my photos in the LCD) and adjust my settings accordingly. If I’m not getting steady shots or I have slowly moving subjects I bump up the shutter speed and see if I can still get exposure ['freezing' people moving around on the Strip will be hard]. If my depth of field is too shallow I tighten the aperture and adjust my ISO or shutter speed to compensate. Etc. Like I said, this method isn’t for high yield, but if you’re trying to squeeze the most out of your camera while handheld I’ve found it the best way for me.
