"Activation Lock for iPhone, iPad, and iPod touch" is a security feature, in part to deter re-selling of stolen devices. To learn more click here --> http://support.apple.com/HT201365
An activation lock can only be removed using the log-in information for the account that applied the lock. If you know this information click on the link to read how to "Turn off Activation Lock." --> https://support.apple.com/HT201441
You are the original owner but you have forgotten the log-in information. Try to recover it:
"If you forgot your Apple ID" [this is likely an email address you are using or used in the past] click here --> https://support.apple.com/HT201354
"If you forgot your Apple ID password" click here --> https://support.apple.com/HT201487
Since you bought the iPad secondhand you do not have the right kind of paperwork to support you asking Apple if they will unlock the device for you.
Unfortunately you may not be able to regain access to the device and it iwll have to be recycled. :-( It is important to keep secure records of log in information because in the digital age these things start to have real monetary value associated with them.