One of the most fundamental things to know in photography is the relationship between aperture and depth of field.
There are two main features that make up depth of field 1. the distance between your subject and your lens and 2. aperture. If you set the lens to the maximum aperture, i.e. its widest setting, you minimise the DOF and in turn setting the lens to a minimal aperture greater enhances the perception of depth in your photo. Small aperture settings also increases the sharpness of the scenery of which you are shooting.
There are no real rules as to what aperture you should use for every shot you take however there are a few generalisations we can make. Landscape photographers more often than not want the whole scenery to be a crisp, clear image so will usually opt for a small aperture, Portrait photographers however will usually use a wide aperture setting so that the subject is crisp and the background is somewhat blurred.
Try out all types of apertures until you're happy with what you've accomplished, as Imogen Cunningham said "Never stop photographing. It is very likely that your best photograph has not yet been captured."
