The basic overview of molding and casting is simple: Take a sculpture, coat it in silicone then create a rigid jacket around the cured silicone. You can then remove the sculpture and fill the void in the silicone with resin. When you peel the silicone off of the resin, you have an exact copy of the original.
Of course, this is easier said than done!
These links below describe the process well:
Make Magazine's guide to Molding and Casting http://blog.makezine.com/archive/2008/07/how_to_mold_making_and_ca.html
Smooth-on's Video Tutorials:
Speaking of Smooth-on, they offer all of the supplies you could need. Reynold's Advanced Materials is the major retail outlet for Smooth-On's products. They have a retail outlet in Brighton Mass, which is a great place to see things in person.