Talking about composition, here are a few pointers that may help you to produce more interesting and varied arrangements for your paintings.
Whether the painting is of an interior or exterior view it's always a good idea to provide variety. In a still life use objects which vary in size and shape. Think of a ground plan of the object group, if you are arranging a still life group, look at it from above and make sure that the distances between the objects are varied.
Painting a scene? Make sure that your close-up features are much larger in scale than those in the mid-distance and the background. Make sure that you have a foreground, a middle ground and a background.
Don't be afraid to use dark tones, pale watercolours lack substance, use all your tones and plan the distances between them. Sometimes you can do this better by judging your painting upside down. Your greatest differences in tone can be used to highlight the centre of interest.
If you use all bright colours in your painting they will fight each other for attention, large areas of muted colour towards the edges of your composition will enable you to use your brightest and most garish colours and contrasts to draw attention to where you want it to go.
Make sure that if your picture has a preponderance of horizontal lines and shapes that you have enough verticals as well for variety. Conversely enough horizontal lines in a picture of predominant verticals.
Leave vignetting to the photographers.