Automatically filling fields with 'standing' data

I think this should be easy but cannot get my head round it.
I have a database that includes a table of clients and a table of contacts.
Each contact is linked to one client.
When I input a new contact (John Smith say), into the contact table and
select from my pull down menu which client he is linked with (ABC Ltd, say),
from the client table, I want the address and contact details of that client
to automatically fill, in the John Smith contact form.

The relevant information resides in the Client table

I cannot seem to be able to make this link