I started using blender some weeks ago and this are my results. Please rate them and give me some advice how I can improve my skills.

meh, the first is the only "original" one, the rest just look like random crap you pulled out of your ass :P

6/10 you need more practice bro. nice lighting though.

the second looks good aswell.
No big deal, but looks nice ;P

3rd.
Light should apply on the piles too.

I think you should pimp it up with ambient occlusion. It makes your scene look really cool instantly (with the expense of render time, and below 9 samples it looks bad, so increase it from the default 5) and maybe mix it with a ramir with gloss value less than 1, making blurred reflections. Well, the latter takes extremely long to render in combination with ambient occlusion, but again, it will look awesome, since it is realistic that there is some global illumination and not for it to be pitch dark in shadows.

Read the rules.