NUMBERS FORMULA IDENTIFICATION?

Apple Computer Problems led to the current installation of Numbers. I am totally unfamiliar with Numbers. I want to set up Tables(?) providing each months church expenses. And for each month, I want to have a column that provides a running total of each expense category. To do so I need to set up a formula to "automatically" access each previous month running totals, by category, and add the current month category to it for a current running total. HELP! Thanks.