Lynn,
Are you on a Linux server? I think you are talking about using a mod_rewrite solution which, I think, only works on Linux.
mod_rewrite is incredibly powerful and fussy. I wouldn't risk advising you on the actual script.
I actually think this kind of question might best be handled by the support team at your server company. I'm not saying you shouldn't ask here... but if you don't get a response that helps, that is another route to go.
Shai