I've had a couple people comment to me that they find low light situations confusing. That's understandable. All the factors that come into play in low light situations can make solving the problem difficult. It should be simple, get more light into the camera. How you do that is the hard part.

You need to first ask yourself two questions; am I photographing something moving, or am I photographing something stationary? Your options depend on your answer.

For stationary subjects you don't have to worry about your shutter speed but if you're going to shoot any slower than your focal length, be sure to use a tripod! (see below for an explanation of shooting slower than your focal length).

Now, for the clearest images, use your highest ISO (100 on most cameras, 50 if you've got it). Depending on your subject you'll need to decide how much DOF (Depth of Field) you'll need and set your aperture accordingly. Remember, while an F-Stop of 1.2 will let in a lot of light, you'll have a very narrow Depth of Field (See my DOF article for more info). At F16 pretty much everything to infinity will be in focus, but you'll need a lot of light.

For moving subjects you may have no choice but to miss some great shots. That's because you'll be very restricted in the options open to you.

The first problem is that you need a fast shutter speed to prevent blurred shots from hand shake or object motion. Secondly, if you go with an F-Stop of 2.8, your subject could move out of the focal plane by the time you actually take your shot, ending up in another blurry photo. That leaves only one other option, increase your ISO to 400, 800 or higher! This can be okay with film, it can even add character to images, but with digital it can make your shots useless. The increased noise in digital images is often more unpleasant than just getting rid of the shot. It's judgment call you'll have to make.