How do I create a list of costs and get subtotals of each category
 

Im trying to organise a list of costs that our business has month by month.
This i have done using 12 sheets (months) and colums to list the items with
their date and cost. So i can see total outgoings each month. However, i
would like to be able to categorise these costs (eg Postal costs, Utility
Bills, etc) to find out the monthly total spend on each category. I am having
trouble creating a drop down list of categories to select for each entry and
then working out (once thats done) how to get subtotals of the items in
particular categories. Any help or suggestions would be much appreciated!