I don't know what you have tried so far, or what your level of Tableau knowledge is, so I'm assuming you might be quite new to the product. Stripping back your requirements, you can get the % difference from previous month using a quick table calculation (Quick Table Calculations - Tableau ). A property of this is the 'relative to' field which by default is set to Previous
To get the difference from the same month in the previous year, I would tackle in a couple of steps.
1) use a calculated field to get the sales for the record 12mths ago
2) create the % diff calc based on this
Using Superstore data as an example, this tries to show what these calcs are doing.
Packaged workbook in v10.1 attached with the above. Hopefully it will set you on the right path. It's likely you'll need to provide a packaged workbook, if you're still struggling after this.(Packaged workbooks: when, why, how )
ForumQ.twbx 393.8 KB