Type "cube(10);" in the edit box to the left.
In the menu select:
Design-> Compile and Render (CGAL) or press F6
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: