Simple cube?

This must be a very basic stuff - but I can't figure out how to make a cube in a right way.

So I did the following:

1) Made a square
2) Extruded it
3) And here I stuck, since I wanted it to be extruded to the exact length of the base square's side,
so I could change its length and have my cube size properly recalculated.

Could you point out to the right tutorial/webinar/whatever please?

Thanks a lot!