Simple newbie(ish) question

I have a quick question about Access forms which I'm
hoping someone can help me with! I have a set of tables,
one for agencies and another for clients. Each client in
their table has an agency name against them, and the
agents in the agency table are set as a primary key, with
a relationship.

I would like to create a form where the user can select an
agency from a combobox, and the list of clients linked to
that agency would populate a listbox next to it. I am
told that I need to bind the controls of the form, but I
can't seem to find any pointers on how to do this. Any
help on where to start???