2 combo boxes in one form

Hi,
I need to make a form with 2 combo boxes and then to fill all other fields
on form, which are text fields. My database has 3 tables. Row Source for my
first combo box is from one table, Row Source for second combo box is from
second table, but this selection should be limited with choice in the first
combo box. When I choose in the second combo box, all text fields should fill
automatically.
All 3 tables are connected with relationship.
How to do this? I suppose that I shoud use AfterUpdate procedure on first
combo box, but how???