An Inventory order and tracking db for hydraulic fittings

I am starting a new db to track hydraulic fittings and inventory. I want to
do it right, from the start by having it normalized, and not quite sure what
I am doing.
My employer and head office is use to the old part # of the fitting and
wants to use it for billing and organization on the parts shelves, but the
parts have to be ordered using a new # part # because they changed
suppliers. Yes they should use the new #'s but they don't want to and they
sign my pay cheque, so I have to try to keep everyone happy.
Each fitting comes in different sizes, and is going to have an old part #
and an order part #. I think it should have an inventory amount, a sold
amount for the month / year, and amount ordered for the month / year.
I am not sure how many tables I will need to accomplish this and what
fields I should have in each table.
Can you send me in the right direction

Thanks Blair