What program do you use? Could a normal computer savvy person figure it out or does it take a class or 2?
Most 3D printers require STL files, which most 3D software programs use. They accept other file formats also. I don't know much about 3D rendering, but I know that if the object isn't made properly you can get into manifold errors and they won't print. If something isn't shelled right, or has one manifold error, then they can't print it.
I made this art specifications file in Illustrator, and my friend I went to art school with is rendering it in 3D for me. He probably uses Maya. I probably could have figured out how to do it myself, but he has a degree in 3D design. Why not let the pro do it? A lot of people can fumble their way through Photoshop too. But I get paid to use photoshop and illustrator because I have a degree in graphic design and I use the programs every day.
If you want to take a stab at it, google sketch up is free (I played with it for about 30 minutes one time) and you can download it on the net. If you run into problems shapeways.com has a forum and people offer a lot of help there. Kind of like this place.