Using the SQL date function to find aggregate totals by month

The only real problem here is which date function you can use. Date functions actually precede the adoption of SQL Standards for temporal datatypes, so many database systems have their own, proprietary functions. In Standard SQL, the function you need here is EXTRACT.

SELECT EXTRACT(MONTH FROM salesdate) AS salesmonth
     , salesperson
     , SUM(salesqty) AS total_sales
  FROM widget_sales
GROUP
    BY EXTRACT(MONTH FROM salesdate) 
     , salesperson
ORDER
    BY EXTRACT(MONTH FROM salesdate)
     , total_sales DESC

This query will combine monthly sales for all years, if the table contains data for more than one year (although your question seems to suggest that it doesn't).

Notice that the ORDER BY clause sorts the results into months, and then within each month, by decreasing total sales.