there are a number of ways to approach this that i can think of.
Do you have a definitive list of products? does the name ever change, and by change i am talking about spelling mistakes.
Simplest approach i can think of is 13 worksheets, 12 are simply copy and paste of each monthly sheet.
The 13th is where the totals are, the first column is a list of all products, across the top give each column a month as a header.
then use a vlookup to populate each product for each month.
Then if you add a total to the last column on worksheet 13 that will show how much of each product you have sold each month. But for this to work the product name must not change.