Practice, practice, and more practice.
In addition to shutter, aperture and ISO, there may be flash.
Very briefly, aperture controls depth of field and in the case of flash, light from the flash; shutter controls movement and in the case of flash, ambient light; ISO controls sensitivity and image noise.
Small f stop numbers are large apertures, large f stop numbers are small apertures because the number is the denominator of a fraction. 1/2 is larger than 1/4, etc.
Faster shutter speeds, smaller apertures and smaller ISO numbers all result in less exposure; slower shutter speeds, larger apertures and higher ISO numbers all result in more exposure.
There is no "right" answer except a photo you are happy with. There may be many combinations that provide an acceptable image, though the characteristics may differ depending on the combination used.
Explore lighting from the front, from the side, from the back, and from combinations of front, side and back.
Practice some more. Enjoy! If something is unclear, try to articulate the question, someone here will try to provide an answer, sometimes many people will.
This was help to me as well, thanks Camera Clicker
Go to places like youtube.com or adoramatv for tons of free tutorials. Watch your in-camera meter to make sure you're exposed correctly. This may mean that you need to compensate (look up exposure compensation in the manual for your camera).
Enter this search parameter string into Google:
exposure triangle iso aperture shutter speed
You should be up and running in no time. Shoot in RAW mode if your camera supports it. almost anything can be fixed in postprocessing.
Experiment. It's free (assuming you shoot digitally, not film).
they don't really affect the lighting, just the exposure. Forget all the math, unless you have a very old analogue camera.
if you want a blurry background and foreground, use larger aperture.
if you want less blurry background/foreground, use smaller aperture
if you want less blurry background/foreground, but don't have a lot of light, you can decrease shutterspeed or use higher ISO
if you want less blurry background/foreground, and fast shutterspeed (to avoid camerashake) use high ISO
if you have very little light, use large aperture and high ISO
Unless you're on a tripod or in a studio, no need for manual setting. Just set the camera on aperture priority (Av), and you only have to consider how much blur you want the background/foreground. The camera will adjust the shutterspeed down to 1/60sec, and if that's not enough to get enough light, it will set a higher ISO.
The body does not matter, assuming the meter is accurate. The ISO setting, aperture and how much light is present determines shutter speed, if you are in aperture priority mode.
If you have the aperture set around f/4, and a shutter speed is 6 seconds, you have very little light. Try raising ISO to 1600 or 3200.
Changing ISO from 100 to 200 is like opening the aperture a stop. If you have everything set to manual and don't change anything else, the picture gets more exposure. As you get further away from the sensor's native sensitivity, electrical noise increases. This noise appears as coloured grain, or little specs, in the photo. Different bodies have different ISO ranges and different amounts of noise at any given ISO setting.
Yes, for any given focal length, f/4 will have less depth of field than f/9, when focused at the same distance. Also, there is a hyperfocal distance for each aperture, at the same focal length. For instance the hyperfocal distance is 37 feet for 55 mm at f/9 , giving 18.5 feet to infinity, and 82.9 feet at f/4, giving 41.5 feet to infinity.
If I may, all the above information is helpful; add to this, pay attention to your in-camera light meter.
Start with either Manual, Aperture priority, or Shutter priority, and learn how your images are exposed using different combinations of the settings...
A good starting point, depending on the type of shots you are after of course, is either choosing a low ISO (1-200) and try some shots in A(aperture) or S(shutter) priority.
Also, browse around nikonians.org, since you own a nikon, and think about a membership there...some good blogs, podcasts and forums
Using a light meter to take readings is going to be of great help to you.
Good information so far. something to REALLY watch is the setting you have on your light meter. For low light situations, a spot meter is essential. read the area you WANT to see. Remember a light meter will under expose white and over expose dark. You need to make adjustments accordingly. I do NOT recommend automatic settings on a camera for low light exposure. Sometimes auto works, sometimes it doesn't. By taking control, you can get the best results possible for the conditions.
oh.. watch the auto focus too. It takes light for auto focus and the focus assist light does not work at decent distance.