One way to learn, is to tke the advice here, and make a sample scene of your model; couple of walls with a window, portion of roof, yard, driveway, and trees. Small to cut down render time. Set your render settings for speed, and do a bunch of quick changes. Work on one thing at a time, like the grass. Try different textures, scale it, rotate and recolor. At first, don't change many things at once, you won't know what is making the difference. In the hands of the right person, even the very basic stuff in SU will often do, but there are times you have to get better textures. Do this to get a feel, don't try for perfection, everything will change when rendering the big model.
The glow around the trees may be a problem with the renderers alpha support. If so, you have to get better, or 3d trees. Take out the gray looking trees, they don't match the composition. For variety, scale, rotate, skew, combine, recolor, and flip the trees. And of course, you got to get shadows.
A good rendering is as much about composition as it is about render materials. Try different angles and views. 2d components look best when viewed at eye level. If it doesn't look OK in SU, the render will not help. Often post processing makes a huge difference, still, its best to first depend on SU, then the renderers, and only if necessary a post processor. Oh, at first, plan to spend at least 4 time as much time as you might think you take.
