Now  select the top of the wall by holding your left SHIFT key and press your left mouse button

once its selected press E and mitre the corners of the wall should look like this...

now press Esc to deselect and continue to do this to the 3 walls you now have left.. 

should look like this..

in the top left hand corner of the grid you will see   (XY  Top) this is your position looking down on your map....we change position by holding  LEFT ctrl and then pressing  TAB  (press tab once)

do this now and select the floor and move it down 1 unit (square)

do the same with the ceiling but move this up 1 unit  (square)

We now have a basic hollow block