SELECT * FROM orders WHERE OrderDate >= DATE_FORMAT(NOW() ,'%Y-%m-01')
Would give you sales from the first day of this month... but really there are a dozen different ways it could be done... for instance:
SELECT * FROM orders WHERE Month(OrderDate) = Month(Now()) AND Year(OrderDate) = Year(Now())
mySQL has quite a few different functions for manipulating dates. Take a look at: date-and-time-functions.html
using those creatively should allow you to do whatever you need to.