I worked in Blender, which can be downloaded for free. It can be intimidating at first, but I've found tutorials are plentiful!
I typically work using Blender Units as Centimeters. Per the contest rules, the ornament could only be 3” by 3” which meant 7.6 centimeters by 7.6 centimeters.
I don't own a 3D printer, so I typically use companies like Shapeways to do my prints. Shapeways has start of the art 3D printers and can print in a variety of materials, including premium metals like gold and silver. I reviewed the Materials design requirements and decided that I would want my ornament to be 1 millimeter thick—which would keep it printable in the materials I was eyeing-- Strong & Flexible Plastic, Brass, and Premium Silver.
Deciding your materials and knowing their limitations from the get-go is really helpful and makes the process go MUCH smoother.