Tinkercad -- A free, web-based 3D design software by Autodesk. Tinkercad has an extremely simple interface, and is a good place to start for those with little 3D Design experience
Blender -- An open source 3D Design software. Blender is more sophisticated than Tinkercad and requires a higher learning curve, but the software is powerful, and Blender offers design tutorials on their website
Shapeways -- Shapeways is a New York-based 3D printing company that lets users submit 3D printing orders in a wide range of materials. Have a piece of jewelry you've designed, or want to design? You can send it to Shapeways, who will print it in plastic, stainless steel, or even gold.