Here is a very basic overview of how to negotiate between your ISO, aperture, and shutter speed settings on your digital SLR camera.
For most new users of digital SLR cameras the different settings can be difficult to navigate. The most basic elements of capturing an image stand out as the ISO, the aperture, and the shutter speed. These three settings will negotiate the speed at which the image is captured, the quality of the image, and how much light is let in. Though you will really have to play with different settings of each in combination with each other to get a clear understanding of how they will work, you still need to get a sense of what each one does and why you would change it.
The aperture, or the iris, essentially determines how much light is going to be let in. Logically, the more you open it up the more light is let in and the more you close it the less light is let in. It then serves to reason that you respond to lower light by opening it up more and you work to close it off when the light is overpowering. In digital photography, the aperture is going to be measured in F-Stops, which you will see listed on your camera's digital readout when you are changing it. The lower the F-Stop number, the larger the opening is for the aperture. For example, f/22 is going to be a very shut-off aperture used only when using your digital camera in a high light environment. An aperture setting of f/2.8 is going to be used only in low light when trying to capture your photo.
The shutter on the camera is what controls how much light is allowed to expose the film or, in the case of digital photography, how much light is allowed to hit the sensor. The shutter speed then refers to how fast the shutter snaps open for light, which you can actually here by the "clicking" your digital SLR camera makes. The slower the shutter speed the more light is let in, but also the less tolerant of motion the camera image will be. If you have a slow shutter speed set, you can expect a lot of blur from the motion of objects, including your holding of the camera. If you have a high shutter speed you will have a much more stable and clear image that will allow you to freeze motion, but it will also be much darker. If you set the digital SLR camera at a shutter speed of 1/60 or slower then you really should be using a tripod or steady surface otherwise you will get blur.
The ISO in digital photography refers to the overall digital camera chip sensitivity to light. In general, the smaller the ISO setting in digital photography the less light sensitive the sensor will be, and the higher the quality of the image will be. In reverse this means that a very high ISO setting will be incredibly sensitive to light, but will also be fairly low quality. This means that you will invite grain into your digital image. Commonly the ISO settings can range from 50 to 6400, and oftentimes people find an ISO range of 400 to 800 a comfortable fit. If you are outside you will not want to increase the ISO above 400, and you could go even up to 1600 or more for dark indoor shots. If the ISO is up this high you can expect a larger amount of grain.
What you want is an image that is going to be exposed correctly, have enough light, and usually avoid a low image quality. This means you have to negotiate the settings for the ISO, shutter, and aperture so that your final image is one that is intended. This is going to mean a special focus on light, so you will have to make sure that the three sets of settings are going to work together. This means that you want to be able to match what your light meter is telling you so that your exposure is correct. For example, if you find that your reading is to set your ISO at 400, your F-Stop at f/5.6, and your shutter speed at 1/60 and you decide to up your F-Stop to f/5.6 you are going to want to bring your shutter speed down to 1/15 to get the right light. If you do this you are going to find that you will get a lot more blur, so you will have to stabilize your image before shooting. You should try to find somewhere in the middle for both the ISO and the shutter speed, and the aperture should be the first place you adjust to match the available light.