If you have an account with Telkomsa you can monitor your usage at http://adsl.telkomsa.net/cgi-bin/onlineGraph.cgi by typing in your username and password. If your account is with any other ISP you should go to http://userstats.adsl.saix.net/. At both you should be able to register to get email updates on your usage.
To monitor your usage on your own, I highly recommend you get a copy of DUMeter http://www.dumeter.com/ - it's a little ap which runs in the background and gives very accurate reports of traffic usage.