Assuming that your table has data for past invoice dates, you can start with something like this:
WHERE [Invoice Date] >= DATEADD(month, DATEDIFF(month, 0, getdate()), 0)
AND [Invoice Date] <= getdate()
You'd have to use GROUP BY if you want to sum the exprices by some other column.
You might also have to either convert [Invoice Date] to the datetime data type, or convert the getdate() to YYYYMMDD if you are storing the invoice dates as YYYYMMDD strings.