How do I find out who using my IP Address ?

[lala]

I really need to know if anyone using my IP Address somewhere else?

[webjnke1]

The only way they could is if they're using your wifi. Best thing to do is change the password. Put one on it if you don't have one.

[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.

[Krowten Nimda]

IP addresses have to be globally unique on the Internet, so no one is using your IP address, unless they are connecting through local network. If you are using WiFi, just make it secure. Read the manual, configure encryption (preferably WPA) with a strong key, and change the default password to login to the router itself.