Navigate to Orders > Customers
Then click the 'Export' button.
This will generate a .csv file, which can be opened in Microsoft Excel, or other spreadsheet program. (all screenshots are of Excel 2007)
You will start by highlighting the last column, titled 'Total Purchase'
Holding down the mouse button, you will drag from right to left, selecting all the columns.
Next you will Navigate to the 'Sort & Filter' option, and select 'Sort from Largest to Smallest'
This will reorganize the rows of customer information based on the total amount purchased, in descending order.
You now can determine who the customers are that have spent the largest amount of money at your shop.
Note: It is very important to highlight and select the columns from the 'Total Purchase' column first, as this is the column we wish to apply the filter to, and it is also important to select all the columns, otherwise only the 'Total Purchase' column will reorganize and the rest will remain static.