Find My iPhone Activation Lock - http://support.apple.com/kb/HT201365 - This is a security feature, presumably in part to deter re-selling of stolen devices.
Turn off Find My iPhone Activation Lock (formerly: Activation Lock: Removing a device from a previous owner’s account) - https://support.apple.com/HT201441
If you really cannot remember any of the identifying information then you are pretty much in the position of somebody who may have stolen a phone. Apple may remove a lock if you can provide sufficient evidence of valid ownership such as providing the original sales receipt, preferably one with your name on it. You must take the device to an Apple Store along with required documentation. There is apparently no set policy (I asked at an Apple Store) and a store may simply refuse, particularly if they are not satisfied you have proven ownership.
Realize this is the digital age and things like passwords and account names need to be taken seriously as they can be involved with things worth thousands of dollars.