So learn to use angles and natural lighting. If you are shooting someone, try to angle the face so that one side is always brighter than the other side.
If you have a SLR, make use of the settings, increase your ISO. Increasing the ISO will make the photos grainy, however. You can use noise reduction software afterwards to reduce the graininess in your photos. Or just use the noise as part of the photo to create the mood.
Another good way to shoot in low light is to use a fast lens. I have bought this canon F1.8 50mm for less than $200. It is a cheap and good solution for low light situations. Anyway, the best is to have a tripod with you, but in case you are not carrying one, you can still shoot nice photographs even in low light.