Help: Access2k

I'm trying to set up a contact database for a salesperson who is
canvassing businesses in certain areas. I have three tables.... Main,
Suburbs, Streets

Streets is basically just a list of relevant street names and an
autonumber ID as the primary key.

Suburbs is a list of suburbs with a autonumber ID as the primary key
and two columns of postcodes (the first being for street addresses and
the second being for postal boxes)

Main is where most of the contact information is stored... company
name, fax number etc. I have drop-down combo boxes for street name and
suburb which work perfectly well. However, I want the postcode to be
automatically looked up and entered into the main table when I select
the suburb. I've tried searching and found several sites offering help
but they all seem to skim over things and I find myself lost in no
time at all.

I also have a form to ease entry of data into the fields in the Main
table.

I'm using Access 2000 and if anyone could possibly explain just what I
need to do in clear, precise instructions it'd be appreciated.
Alternatively, I could zip the file and email it if anyone could
possibly fix it and email it back - preferably with explanation of
what you've done so I can follow the changes.