Just create subsummary parts on the layout by sales month and drop the summary fields into them and then sort by month. You just need a field of Month(SalesDate) that your subsummary is based on. You can then format the sale date field to show only the month name with the figures next to it.
I'm really new at this, is it possible to give me an example/sample. If you cannot, can you give me the name of someone who can provides this service for a fee.
Using the New Layout dialogs will walk you thru to process of creating a subsummary report if you select the New Layout > and chose Report with grouped data.
It's still up to you how to group it, and, as Lemmtech indicated, you will need an existing break field (Month or Month-Year) on which to sort so that your groups break out correctly.
FileMaker's built-in Help has some pointers on creating sub-summary reports to help get you started. This is an extremely common need in FileMaker, so it's worth learning the basics of how to do it yourself as needed.
A lot of third-party FM how-to books have entire chapters/sections on sub-summary reporting.
I agree with Stephen FileMaker's documentation does a good job of explaining the first step is to create the fields on which the summaries will be based once you do that the rest is pretty easy. Just remember to sort on that field to see the groupings properly.