I did a sun and clouds in another wallpaper, and they came out well. However, I screwed up in another part of the wallpaper.
Anyway, it looks simple. That's an of-course. If you use the ellipse tool, and make the color of the circle nearly white with some yellow, after that, just use the blending options to make an inner and an outer glow of the same color. Clouds are pretty simple. If you want to give it a little perspective, just transform with perspective. Then make sure you have a background color you like on a separate layer below the cloud layer. Set the blending mode of the cloud layer to lighten or screen, and duplicate the cloud layer as much times as you want to get the desired effect. That's all.
Overall, it looks a little too simple, and I know you have lots of room for improvement. If you take my advice above, your wallpaper will look a lot better. :)
