If you go to the properties menu for your connection, it will tell you the number of bytes that have been sent and received on that connection. Sometimes there are other connections going on in the background that aren't reported here and the true number can be found in your command prompt.. type in
and look for the bytes section (should be the top number).. to get the total number of bytes sent, add the 2 together and divide by 1,048,576 (number of bytes in 1 megabyte).. that will tell you how many MB.. or 1,073,741,824 to determine the number of GB..
however, this will tell you the total amount sent / recieved while your connection has been active.. this number will reset when you restart your computer or lose the connection in anyway.
There may be software out there that will benchmark these numbers for you to see how much you use in a month, but you may just have to google that unless someone else knows of a good legit program for it.