Google Sketch-up is by far the best CAD program to begin with. I have access to most of the programs. Sketchup also has excellent tutorials on one of their web sites. See the "3d warehouse" for drawings.
Use the plug-in at http://www.cerebralmeltdown.com/SketchupPlugins to convert the .skb files to .stl files on the free version of Google Sketch-up. There is a plug-in and tutorial to make gears at cerebalmeltdown as well. It works great. Thou shout learn to use a spreed sheet to enter the proper dimensions for making gears. Also remember the extrusion is .4mm or about .016 thousands of a inch. and the extruder goes down the center of the g-code. So you have to adjust the inside diameter of holes, up by .016 inches to get it to come out correctly.
Open Office has a free spreed sheet program.