Bandwidth usage

I have a problem with using too much bandwidth (or something) 

I can't get unlimited usage where I live (in the country, in Canada).  I have a limit of 30Gig a month.  I keep going over and it is expensive. 

I don't play games online and rarely even look at the U-tube stuff people post on FB. My son does both but it varies on a daily basis.  We have a router with a password so I don't think anyone is using my hours but.....

Something is using the time though.  Yesterday I turned off my computer after I finished teaching and left it off for the rest of the day.  I used less 1.12 GB vs 2.08 GB two days before. 

What can I check to see what is going online that I am not aware of?  I have already most of my GB for the month (I have 6GB left and it is only the 19th).  Extra charges were $20.00 CDN last month.      

HELP 

http://www.wireshark.org/

Not sure how deep you want to delve into this but this what I have used in the past.

If you're running Windows I think you'll also need to install this
http://www.winpcap.org/

Wireshark is a pretty serious tool. I used to run it across a network of over 150 boxes running multiple OS's. It may or may not be worth it for you. You can find a bunch of tutorials on the web for it.

PS...what form of encryption are you using on your wireless access point if it's enabled....and are you using the default login/password on your router?