Cascade Drop Down Box

I have created a form, with a subform, and created a combo box based on a
query.

My two tables are Customer & Contact. Customer being the one and contact
being the many, the drop down box is based on a query that has the company
name, first name and last name of the contact. But we have a great number of
customers, and i read somewhere that you can create a cascading drop down box
to select them in alphabetical order so I pick A for example, and then go to
ABC Company instead of going through the whole list of companies to find the
one i want.

I am stuck on how to create this feature. I would appreciate any help that
you can give me, or if you know where i can read up on this feature.

Thank you.