Before passing-on the device to you, the previous owner should have prepared it properly for reuse. This would deactivate the Activation Lock and reset the device:
https://support.apple.com/en-gb/HT201351
Here is Apple’s support documentation about Activation Lock - and the methods by which it can be switched-off:
https://support.apple.com/en-us/HT201365
https://support.apple.com/en-gb/HT201441
As you will discover, unless you can secure the cooperation of the previous owner, you’ll not be able to disable, bypass or otherwise defeat the Activation Lock - and the device will not be usable by you or anyone else. Unless you have documentation that proves you to be the original owner (such as the original sales invoice that shows the device serial number), Apple will not assist you. Nobody here will be able to help.