Running Totals

As a very raw beginner with Excel I would appreciate how to insert monthly running totals. I have been made treasurer of an organisation and need to keep the accounts up to date. Attached is a sample file of my attempts and I would like to know how to carry last month's totals (row 29) and add them to this month;s totals (row 28). Each month I create a new spreadsheet by clearing the contents of the previous month's figures (except the formulae cells) and saving the sheet with the next month's name. By opening the old month and the new month side by side I can achieve the desired result by clicking = on the previous month's cell and clicking on the identical cell in the new spreadsheet and pressing ENTER. I then adjust the formula as necessary. I think this is clumsy and time consuming. Can you show me a better way? Thanks.