While I'm no expert on this, I can point you to a guy who is - Neil Blevins. I've learned probably half of what I know from his tutorials and articles on how to make stuff look real in the 3D world. Start here:
This will get you started on making the objects in your scene look less perfect. Real world stuff is beatup, dinged, dirty, and usually has multiple layers of dust, stains, paint, cracks, etc.. Creating the reality will require you to go into Photoshop and make grunge maps, layers of "reality" that will allow you to distort, disfigure, grungify (is that a word?), and stain your perfect world.
In 3D Studio, do the lengthy tutorial on Unwrap UVW; this will get you going on how to paint custom maps that will put the dents and stains in the right places. Work your way through Neil Blevins site, and you'll wind up a real pro at all aspects of making 3D look real. Good luck!