Using Views to filter the content based on the current month
In the latest versions of Views, you can use a date argument to do this.  For example, you could select the node post date as a date to use with the argument, and then if you passed 2011-12 as the argument, it would limit the display to only nodes posted in December 2011.  You can use any ISO date for this, too, so P1M, which means a period of 1 month, would create an argument for the current month.  Check out http://en.wikipedia.org/wiki/ISO_8601 for an indepth explanation of a lot of different ISO date formats.  All should work here, the only one you have to be careful of is the interval format; with drupal you need to use -- instead of / to separate them for hopefully obvious reasons.

If, however, you are wanting to group or aggregate your content by month, check out http://drupal.org/node/388140.