The big three

Balance in ISO, shutter speed, and aperture. ISO is how sensitive your camera is to light, shutter speed is how long it takes for your camera to take a picture (which in turn alters the amount of light coming in), and aperture is how dilated the lens of your camera is. Not all cameras have this, mostly only digital photography cameras. By balancing these and keeping them as close to the middle as possible, you can avoid the noise caused by high ISO, the blurriness caused by low shutter speed, and the depth of field side effects caused by low aperture. Depending on how your picture should be, you should change these settings accordingly to keep light at a good level but still have the effects you want on your picture. For example, say you are taking a picture of a cool bird coming out of the water. You will need a high shutter speed to get it in focus, but you will also need a low aperture or high ISO to compensate for the lighting. A high ISO will make it look grainy, but a low aperture is perfect because it also creates a cool blurry background effect that draws attention to the bird. By balancing these elements, you can make the best image possible.