There are a variety of ways to scan objects for 3d printing. This is not always an easy process and can require a lot of post processing using a large variety of applications.
The most common methods for 3d scanning are:
Hardware Required: Digital Camera
Creates 3D models from series of photographs (Up to 40) taken at various angles using photogrammetry. Photos can be taken with any camera and are uploaded to Autodesk 123D's server for processing.
OS: Windows (Bootcamp Windows works on Mac)
Hardware Required: Microsoft Kinect or Asus Xtion
ReconstructMe is an easy to use real-time 3D reconstruction system using cheap 3d scanners connected to a windows computer.
Hardware Required: webcam, 2 white boards, hand-held laser line
DAVID Laserscanner is a software package for low-cost 3D laser scanning. It allows scanning and digitizing of three-dimensional objects using a camera (e.g. a web cam), a hand-held line laser (i.e. one that projects a line, not just a point), and two plain boards in the background. Its most distinctive feature is that the laser line can simply be swept over the object by hand (like a virtual brush) until the results are satisfactory. At the same time, DAVID Laserscanner generates 3D data in real time and shows them on the computer screen.
The resulting 3D mesh can be exported into well-known file formats and can thus be imported and processed in most 3D applications. The software is also able to grab the texture and "stitch together" scans made from different viewing directions.
3D Scanning Cleanup
Good tutorial going over the workflow required to cleanup some 3d scans.