Unless you enabled Find My iPhone on it before it was stolen then there isn't any way to locate it. If you did enable it then you could try locating it either via http://icloud.com on a computer or Find My iPhone / Find My iPad on another device - but that will only work if it's connected to a network and the device hasn't already been wiped and/or Find My iPhone disabled on it.
If you haven't already done so then you should report it to the police, and contact your carrier. You should also change your iTunes account password, your email account passwords, and any passwords that you'd stored on websites/emails/notes etc.
Stolen products : http://support.apple.com/kb/ht2526