Tutorial: Create a 3D Printable cube with OpenSCAD

Download, install and launch OpenSCAD.

Type "cube(10);" in the edit box to the left.

In the menu select:
Design-> Compile and Render (CGAL) or press F6

and then:
Design-> Export as .STL

Give your file a suitable name e.g." my_cube.stl"

Print the file on your 3D Printer by loading the .stl file in you printer software.

Here is a good source to learn more about OpenSCAD:
http://edutechwiki.unige.ch/en/OpenScad_beginners_tutorial