Select 1 Pce Of Data And It Automatically Selects And Inputs Relating Data In A Form

i would like in a form for a combo box to be able to select an item from a table and input relating information automatically into other boxes in the form..

I have 3 tables: Table 1 has product code and product description.
Table 2 has invoice number company details, address etc.
Table 3 has product code and product description qty and invoice number..
Table 3 relates to table 2 by the invoice number and table 3 product code looks up the product codes available in table 1 and also table 3 looks up the list of products descriptions in table 1 using the combo wizard.
This means the wrong code can be put with wrong description.
What i would like to know is how i select a product description and the product code in the form fills out automatically?? i hope this makes sense please helppppp!!