Since you want to summarize the data in the table by a date period, you will
need to use a Total query. Since you want to compare the value against
another date range from the same table, you will need to use a subquery to
get the value for the previous period.

Using the Northwind sample database (and ignoring discounts and freight),
this example shows how to get the sales for a month for each product:

SELECT Year([Orders].[OrderDate]) AS TheYear,
Month([Orders].[OrderDate]) AS TheMonth,
[Order Details].ProductID,
Sum([Order Details].[Quantity]*[Order Details].[UnitPrice]) AS Sales
FROM Orders INNER JOIN [Order Details]
ON Orders.OrderID = [Order Details].OrderID
GROUP BY Year([Orders].[OrderDate]),
Month([Orders].[OrderDate]),
[Order Details].ProductID;

Then adding the subquery to get the value of the previous month's sales for
the same product, you end up with:

SELECT Year([Orders].[OrderDate]) AS TheYear,
Month([Orders].[OrderDate]) AS TheMonth,
[Order Details].ProductID,
Sum([Order Details].[Quantity]*[Order Details].[UnitPrice]) AS Sales,
(SELECT Sum([OD].[Quantity]*[OD].[UnitPrice])
 FROM Orders AS O INNER JOIN [Order Details] AS OD
 ON O.OrderID = OD.OrderID
 WHERE (Year(O.OrderDate) = Year([Orders].[OrderDate]))
 AND (Month(O.OrderDate) = Month([Orders].[OrderDate])-1)
 AND (OD.ProductID = [Order Details].ProductID)) AS PriorMonth
FROM Orders INNER JOIN [Order Details]
ON Orders.OrderID = [Order Details].OrderID
GROUP BY Year([Orders].[OrderDate]),
Month([Orders].[OrderDate]),
[Order Details].ProductID;

If subqueries are new, see:
   How to Create and Use Subqueries
at:
   http://support.microsoft.com/?id=209066