Help with spreadsheet functions

I want to assign a function to a field that shows the sum of all my bills, and each time i check off an item, it's value is deducted from the sum. What is the appropriate fx code for this action? I'm new to creating/ using spreadsheets and I'm trying to use the numbers app on my iPhone to create a spreadsheet to track my bills due. It is a simple sum checklist with items, the amount due, and a checkbox beside each item.

You can either just do a simple sum of a list, or set an "IF" function to only total items that match a particular criteria.

Put them all in a row, the one under the next. Once you pay any just move it up one column. (drag and drop).
Peace.