Tinkercad is a beginner level 3D model maker that's not only simple to use but also very easy to learn. Tinkercad is browser based- just create an account and you're ready to go, allows simple exporting of models into .stl files which can then be 3D printed. It is also the only program that offers online help and teaching, which you can find here.
Blender is a FREE to use 3D creation suite. It allows modelling, rigging, animation, simulation, rendering, compositing and motion tracking, even video editing and game creation
You can even make full videos like the one below. It was completely made by the people at Blender Foundation and can be made by every day people
You can find tutorials by Blender here
Autodesk 123D Design is a 3D design app which works in conjunction with Tinkercad and any 3D printer. You can easily import your work to other devices as Autodesk 123D Design is also available on Mac, Windows and Ipads.
123D design offers Video tutorials found here
And a design manual found here
Below you will find a video on different ways you can use 123D design
"Enter a Digital world without Boundaries"
Sculptris allows you to make your own 3D design out of clay. It provides a simple way of molding clay into any shape you can imagine and then simply export it into a 3D printer. Sculptris also allows you to design, sculpt and even paint