I see a lot of inconsistency in your work, but also a lot of potential (a lot of your stuff reminds me of mine/Surf/Cercopes early work).
PM'ing you a site with some useful tutorials and stocks/references. I think you'll pick it up pretty quickly.
Anywho, on to actual C&C- you need to work on lighting and depth. The lighting in most of those is nonexistent-- I suggest you just take a large, soft white brush on where the light source should be and set the layer on Soft Light and reduce the opacity as you see fit. As for depth, it looks like the background is directly under the render, which results in your signatures looking decidedly 2-dimensional. Try duplicating your render and smudging it in the background, then put a copy of the original render over that and erase or blur the edges of it. This will make the render look like part of the background rather than separate from it.