If you're shooting handheld images in low light, there's a simple formula to know.
You need a shutter speed of 1/(focal length) second or faster. So, when using a 100mm lens you need a shutter speed of at least 1/100 second. (The focal length to use here is the 35mm equivalent focal length.)
If light is really low, to get fast enough shutter speed, use a high ISO setting on your digital camera, and a wide aperture. Try to brace the camera against something (a wall, a door, a car) if you can. Press the shutter release smoothly (don’t stab at it).
With a compact digital camera, you may get sharper shots using the optical viewfinder (if it has one), holding the camera firmly up against your eye, than holding it out at arms length and using the LCD for framing.
If it's impossible to get a fast enough sutter speed, try setting the camera on continuous shooting mode and take a series of shots. Though most may be blurred, the chances of getting one sharp image out of many is a lot better than getting one sharp image out of one! The illustration shows this technique when shooting with a 70mm equivalent lens at 1/6 second. That’s about 3.5 stops too slow to be sure of a sharp image, but I got one!
Saved by a sequence: Sharpest and blurriest images from a sequence of eight shot at 1/6 second with a 70mm lens.