Automatically add field data

I'm using MS Access 2000.
I want to be able to put in a piece of text in one field of a table and the database automatically add another field.

For example, enter a supplier and the database adds the catagory.
So we could have 20 suppliers but only 5 catagories. How do I do this?
I can create another table with each supplier matching a catagory.

Supplier1   Cat1
Supplier 2  Cat 1
Supplier 3  Cat 2
etc., etc.,

Thanks for your help. Hope you understood what I am asking! (If this is not possible in Access then I could use Excel.)