Can anyone assist me in my design?

Real world:
I am a manufacturer of goods sold to a national retail chain. I have 10
different items in 1800 stores.
Every week I get a report from the chain that shows inventory levels and #
of purchases that week for each item, each store; 18,000 line on a
spreadsheet.

In Excel :
I have a worksheet "Store List" which has all the information about the
stores; Store number, address, regional info, etc. all pertinent info the
chain uses to describe its stores.
I have a worksheet "Product List" that has all info on each of the 11 items,
SKU number, our part number, size, weight, cost, retail, description
I have a worksheet "Sales Data" that contain a line for every store, every
item... a reflection of the reports received from the chain with an
inventory column, sales column, and on order column for each weeks report I
receive. I transfer over the Inventory and sales data to a new column by
hand.

I have written many functions and acros to analize the data, but it is very
inefficient.
I would like to utilize classes to improve performance and ease of
programming future improvements.

So far I have:

A store class: Describes any one store with all the store data from the
"store list" worksheet
An item class: Describes any one item
A Product Line class: Which creates an array of 10 item classes

Where should I store each stores sales and inventory data?
Should "Qty on hand", "Quantity on order", "Quantity sold" all be private
members of the item class?
Should each weeks sales info be held in an array within the items class or
the store class?
Should the store class have an array of weekly product line snapshots?
Should I be using a database?
Should I go back to college? :)

Any direction will be apreciated.
Thanks