How do I find out who using my IP Address ?

[Forever]

If you have cable internet type 192.168.1.1 into your web browsers address bar. If you have DSL then type in 192.168.1.254

This will bring up the login page for your router. You will need to know your login information if you changed it from the default login info. The default login username is almost always "admin" and the default password is usually "password" or "admin". Once you are logged in you should see a web page that has all kinds of options to configure your router. Somewhere in those tabs will be a list of every device that connects to your network. If someone has been leeching off your internet and therefor using your IP address his device will be listed there.