How to create a 3D object
There are three ways to create virtual 3D objects to print.
First and most difficult is to start from scratch using free computer-aided design/computer-aided manufacturing (CAD/CAM) software such as 123D from AutoDesk (a MakerBot partner), OpenSCAD, Sculptris, GrabCAD, Blender, 3D Tin and Google SketchUp. Once your object is designed, you bring into MakerBot's MakerWare 3D printer-setting software – but this DIY object design creation method is only for the technologically adventurous.
The second way to create a virtual 3D object is to scan an existing object to create a duplicate or a variation on a theme.
See the full story here: http://tech.lifegoesstrong.com/article/everything-about-how-3d-printing-works-part-2-0