Aperture in Composition
Aperture plays a key part in composition and makes a significant difference to the quality of your image. It helps to isolate a subject or sharpen the details across a wide landscape. When combined with the rule of thirds you are able to produce very powerful images from the most simple subject. Wide apertures (small f-stop numbers) tend to work better when your target is a specific thing that is better suited to being isolated such as portraits, wildlife or macro work and is not normally used for landscapes . The gallery below shows the use of a wide aperture to isolate a subject creating pleasing images from fairly routine photographic subjects.
Small apertures (large f-stop numbers) are used to convey greater depth by maintaining the detail throughout an image. So in terms of your composition aperture should be another thing you consider very early on. It's all about what you are trying to convey and how much detail is needed to achieve your goal. Generally a narrow aperture is used for landscapes but not always, as with all tools of composition you only apply it to help you capture what you want to show. The gallery below uses narrow apertures to maintain detail across the images
Obviously there are more aperture settings than wide open or closed tight but I'm using the extremes of the range in this instance to better demonstrate the value of aperture in composition.