I use the low-key setting on my camera and mess with the iso depending on how much light is available in the room/area for the dark effect. After I upload it I tweak the exposure setting through a plug-in with photoshop, it's called "cybia" and it comes with a bunch of cool filters and settings. I also have to mess with the curves sometimes. What kind of camera do you have?
If you can't find a low-key setting under your pre-set options you could also try adjusting the iso under your settings, the lower the number the less sensitive your camera will be to light. I'm not sure where those settings would be with your camera, I've never owned a canon DSLR, but I can't imagine it differing too much from nikon's set-up. Definitely mess around with it and see what different settings/buttons do. I spent years with my camera before finding just the "right" settings. I'm too stubborn to pick up manuals or learn technical terms