Firstly check your ISP's web site..... some have usage stats when you login to your portal. otherwise If you want to keep track of all devices overall usage then your router is the place to look.
Some have fairly simple bandwidth logging on them (you may have to enable it) and some have nothing that will help, so you may have to check.
There is software that claims to make it easier to collect the log details from the router...
http://www.wallwatcher1.com/
(Not tried it myself I use
http://www.clearfound...oftware/overview.html on a separate computer as a gateway)