I'm confused about "Cost of Goods Sold." How is it calculated?

 I can transfer inventory at the end of last year to inventory at the beginning of this year.  I didn't buy anything (business stagnant), so there were no purchases.  There was no labor, but there were finance charges since I bought product on credit cards during previous years.  Does that cost go here?  (Accountant did not do this.)

And then how do I know what current inventory is?  I sold some product, so do I go back and find out my original cost for what I sold and deduct that?

Am I supposed to have a record of all my inventory in QuickBooks?  (I'm trying to take over from the accountant.)