how to order a dataframe ?

Hi,
I've got a problem which seemed simple to me at first view, but which I 
haven't managed to solve yet. I have a dataframe, or a matrix, and I 
would like to order it along with one of the variables/columns. I tried 
to use order() but it remained quite unclear to me. How should I proceed ?
Thanks in advance (I guess that's not very difficult a question for most 
of you)

Laurent