If you had "Find My iPhone" enabled in Settings > iCloud then you may be able to locate it. If not, you cannot locate it.
If it was enabled then you can try locating it via http://icloud.com on a computer or the App "Find My iPhone" on another iDevice. Note: this will only work if your device is connected to a network and the device hasn't already been restored as new and/or "Find My iPhone" has not been disabled on it. Note: Disabling "Find My iPhone" is much more difficult in iOS 7.
If you think that your device was stolen rather than lost then you should report it to the police. In either case you should contact your carrier, change your iTunes account password, your email account passwords, and any passwords that you'd stored on websites/emails/notes etc.
See also here for additional information: http://support.apple.com/kb/HT5668