Date of creation and of change

Hello,

could someone enlighten a fairly beginner how to define columns
of a table with the following features:
  CreateDate DEFAULT value should store current date and time  ChangeDate DEFAULT value at creation as above and a
Trigger            function which stores data end time of any change      to the data set
 

I hope someone has this quite usual feature handy or at least a
pointer where this is described.

Thank you very much
          Andreas.