I can tell.
You can. I'll take apart what I posted.
In your signature, it is more distracting than complimenting. One of the reasons for this is because it is more centered, while your render is off to the side. Try moving your c4d closer to the render, or better yet, center your render, put the c4d on a layer above the render, and play around with some screen/lighten/linear dodge blending settings. Erase what looks bad.
Your text doesn't accomplish this as it sticks out to much. Try setting the text to certain blending layers, using less intricate text, and positioning it closer to your render. Also, it is a little big I think. Don't worry to much about text at the moment, but don't neglect it either. Text is difficult for most people.
Basically restating that you need to make sure things such as text and c4ds don't draw more attention than your render.
Shadows are created, usually, by use of the burn tool(be careful) and black or dark brushing set to things such as soft light or on low opacity.
Lighting is tricky sometimes. You can start it by taking a 100px soft brush, color white, and brushing were you think the light should come from based on the way your render is lit. Then set that layer on soft light, if it should be subtle, or just lowering the opacity some. Then, as you go through making your signature, you can smudge some white brushing to "draw out" the lighting some. I know it sounds like a lot of work, but I personally really like doing lighting.
Overlapping is somewhat obvious. Just don't overdue it and remember you have great blending modes such as overlay and color dodge and an eraser

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I hope that all made sense to you. Just ask if something doesn't and I will explain it a bit better, if I can. I prefer if you ask in threads such as this one so everyone can benefit from the answer. Good luck and good job.