The most readily available form of light is of course natural daylight. It is present everywhere around us, although you should realize that its characteristics can be very different depending on the time of day and the position of the sun.
Midday sun is rather blue and creates dark harsh shadows because it is right above you. Morning and evening light are warmer in color and since the sun is lower in the sky it creates lovely long shadows.
Sometimes however the available light is not enough to create the image you are after, and some form of artificial light will be needed. The most readily available is of course your in camera flash. Although it is rather limited in its coverage as its guide number is usually only enough to cover subjects of about three to four meters away, it can be a very useful tool as a fill-in flash to supplement available light and to lighten up dark shadows in your subject.
Then we have studio flash which is usually only available to professional photographers. These are big flash units with very high guide numbers for use in the studio. They are very versatile because their output of light can be varied in Â½ stops over a wide range.