http://www.apple.com/icloud/find-my-iphone.html
Log in using your regular apple account name and password. If your phone is switched on, you can find it, if not, I believe you can set an alert to tell you when it is switched on. You can also disable it from there, too.
Of course, if you didn't set an icloud account, none of this will work.
I am at a loss if I do t have the phone, LOL. I know it's probably a pretty simple answer!