I use three different programs to design 3D models depending on what it is I want to do.
SketchUp is great for knocking something simple out quickly. You can learn SketchUp by watching their online video tutorials at http://www.sketchup.com/learn/videos?playlist=58 or by viewing those same videos on the SketchUp YouTube channel.
OpenSCAD is useful when...
To learn to use OpenSCAD, you'll probably want to brush up on your programming skills using something like Khan Academy, and then refer to The OpenSCAD Language chapter of the OpenSCAD User Manual. MakerBot also has a great page that explains how to fix OpenSCAD design problems. If you have trouble exporting an STL file, checkout the OpenSCAD User Manual page on STL Import and Export.
123Design is useful when you want to design something that is compatible with something you would find at Lowes (you can import 3D models from McMaster). To learn 123D Design, refer to their online video tutorials.
To access these videos through the Mat-Su School District's network, use the following links:
Cool Things to Print