Executing scripts upon login
I am trying to write a script that will inform a user of any new messages/files that have changed since their last login. I don't need any help with the actual script, I would just like to know where to place the script and what, if any, changes I need to make with the user settings. I'm only interested in applying this script to a single user. Is there any way that I can get it to automatically execute when this user signs in?
Thank you very much in advance for the help.