There are three main settings that affect your exposure:
- shutter speed - it controls how long light reaches your camera's sensor and usually you should not use longer speed than [1/your focal length*crop factor]. If you choose a slower shutter speed, your image may get blurry as the shutter speed helps freezing action.
- aperture - it controls the amount of light coming in through your lens. A fast lens is 1.4 or 2.8.
- ISO - it compensates for light deficiency. The higher the number, the brighter your image will be but at a cost of extra noise on your photograph. Values may be 100, 200, 400, 800, 1600...
Set your camera to Av (aperture priority) mode, manual ISO. Try to opt for wider aperture than to higher ISO. Let your camera adjust between the shots. It works well in constant lighting situations while where light varies, you can try using Manual mode and manual ISO. In this case you need to adjust your aperture and shutter speed manually. Set your White Balance to manual as well.
Not all lenses can produce a sharp image being widest open, so try to use one or two stops further.