Adding relations and listbox to existing form

[Kath]

I am a beginner trying to figure this out.
I have an existing database that I use a form to enter
data to. I want to build a listbox/combo box so I can
prompt the county codes of a certain list of cities
automatically. I want to add this funtion to the form
that is currently being used. Please tell me the steps
and then, if there is any programming, the coding for
this. Thanks so much guys! This will save me a ton of
time!

Kath