Check if manual aperture settings option is available for your camera. Single lens Reflex cameras and Heavy Compact Digital Cameras have this feature. For this look at the command dial on your camera on top left. If it is labeled with an "Av" or "A," then you are able to set the aperture manually. Digital Cameras may hide this feature in a menu system.
It’s important to know how aperture numbering system really works. Aperture is donated with a number value. Smaller the number, wider the aperture ring, thus allowing a lot of light to enter your camera. If it is donated by a large number, the aperture ring is narrow, allowing less light to enter the camera. For example an aperture number of f2.8 is a wider opening than an aperture of f8. In case all other settings are to be kept constant, a picture taken with the aperture at f2.8 will be brighter.
It’s time to adjust the aperture settings on your camera. Use the manual option in your command dial on your camera to see the present aperture settings on your camera. Switch through the numbers, keeping in mind that the smaller number will allow more light to enter the camera. If one is shooting in the fully manual ("M") mode, the shutter speed will not adjust itself with the changes in value of aperture and will stay constant. Take a few snaps with different aperture values to experiment with different settings providing you different results.
If you use the "Av" or "A" setting you will only have to select the aperture you wish to use and the shutter speed will be adjusted automatically. On the other hand using the "Manual" or "M" mode one has to set both the aperture and shutter speed manually on his/her camera.
One more important effect of aperture is its effect on the depth of focus in your pictures. Commonly used in portraiture are telephoto lenses, as having a small aperture number makes the background get blurry, so the foreground subject to be focused is more defined. On the other hand for Photographic Scenery and Landscapes, wide angle lens is a better option because a large aperture value will shift your focus from front to back.