First and foremost, any photographic situation, no matter what requires the 'right' light to give it that something special, but there will be many situations when the 'right' light is just not available.
Outdoors we rely mainly on sunlight or the natural 'ambient' light (that light which is available to us at the time of shoot). We can take advantage of sunlight because it is fairly predictable. Depending on the time of day, or the season, or if the day is bright and sunny, heavily, or lightly overcast. The direction and intensity of light are also very important. Any of these conditions can bring about the right result, depending on your expectations.
Directional light, (where the sun is very low in the sky - early morning or late afternoon) will enhance form and texture on surfaces and give subjects a three-dimensional look.
It is not desirable to take portrait shots outdoors in the middle of the day when the sun is very bright and directly above. This overhead light will cause dark shadows to form under facial features of your subjects, and especially under hats. You can stand your subject/s in the shadow of a tree, or under the overhang of a building (preferred), with faces turned toward the light source. Make sure that the shadow from the tree is a full shadow and not mottled, (half shadow, half sunlight) otherwise that will also cause an undesirable effect as your camera's light meters will become confused.
You may find a situation where the headlights of your car can be used to throw a little light on the subject. Or even a lantern or flashlight may suffice. These create an eerie effect when photographing gravestones at night, or big old gnarly trees.
Outdoors or in, we can certainly make good use of on-camera flash, flash attachments and/or slave flash (this is a wireless flash unit that is activated remotely by either the on-camera flash, or separate flash attachment), or undoubtedly we could get ourselves some pro lighting gear, but that is quite an expense if you're just starting out.
Sometimes the light from the on-camera flash, or a separate flash unit can be a bit severe, for they can cast dark shadows to the back and/or to the side of the subject. You can lessen the severity of the light from the flash unit by diffusing it. This can be done by placing a piece of white tissue or a handkerchief over the flash unit. And to make the light from the flash more agreeable (if you have an SLR camera with a hot shoe), you can attach a sync lead (extension lead from camera hot shoe to flash unit), or bounce the flash off a white wall or ceiling. This will certainly give you a more shadowless lighting situation, but if the wall or ceiling is too far away, then the power and strength of the light may not be sufficient, or bright enough to provide the required amount of light for your subject.
Another way you can achieve the 'right' light indoors, and without the need of flash, is by arranging your subject by an open window or doorway, which is facing in a southerly or northerly direction, depending on if you live in the Northern Hemisphere or the Southern Hemisphere. The light should be bright, but not harsh, and try not to include the window in the shot, as the strong light in the window will provide too much light and probably blacken the scene. So the idea is to use just the light cast from the window and not the window itself.
Another great light source for indoors or out is the humble reflector. You can purchase these through photographic suppliers, or you can easily make them for yourself. They are a simple but effective device that is used to reflect light onto your subject and they basically come in white, silver or gold.
To make one, you only require a piece of whiteboard. White core flute is excellent for this. It is thick, but it is also lightweight and you can cut it to whatever size you want, circular or square and it is available from framing shops.
To make a silver one, just tear off a length of aluminium cooking foil, (if you are a bloke, you may need to buy a roll, other than use up all you have in the house, otherwise you know who might not be too happy) scrunch it up a bit, then unscrunch it again, then stick it onto one side of the board, then you have a convertible, white or silver reflector.
For the gold one, you can purchase from your local newsagent, a sheet of gold gift wrapping foil. It's applied the same way as the cooking foil and it is good for giving a nice glow to the scene and/or skin tones.
If you are taking close-up photographs towards the light, and your subject is in shadow, or there are shadows there that you would like to eliminate, you may need to use a reflector to bounce light into the foreground. A white umbrella is also good for this. And they also come in handy for other situations too. You may have seen those photographers in shopping malls, taking shots of children. They normally use a single flash unit which is bounced off the inside of a white umbrella to provide light for their subjects.
Other very clever items you can use are sun shields, those things you unfurl and fit onto the inside of your windscreen to keep the heat out. They too work very well as reflectors and the fact that they fold up, make them suitable to be stuffed into your camera bag, or backpack.Reflected light can come from other sources too, and sometimes when we least expect it, or want it, for that matter. And to the detriment of what could have been a good image. These unwanted reflections can come from surfaces that are near to our subject, such as brightly coloured walls, greenery, coloured (stained) glass, fields of brightly coloured flowers, etc., so we have to be aware of these dangers, or perhaps you could even put those to good use too, who knows?
Without light, we would not exist and the same of course applies to photography. It relies on the 'right' light source to bring about the "right" exposure every time and pointing the camera in the 'right' direction will also help.