How to get the Activation lock off of an iPad Mini?

Presumably you are being prompted for your AppleID credentials - so as to satisfy the Activation Lock. You will require the AppleID an associated password that was last used to successfully activate the device - which, for the bona-fide owner, should be straightforward.

If you are not the owner of the AppleID, unless you can independently obtain assistance from the previous owner, the iPad is (and will remain) useless to you. If you are able to contact the original owner, they will require this information:

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Here are the Apple support pages that outline the Activation Lock - and the only mechanisms by which it can be satisfied:

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Turn off Activation Lock – Apple Support

As you will discover, unless you are the owner of the AppleID that was last used to activate the device, you’ll not be able to defeat or bypass the Activation Lock. With documentary evidence that you are the original owner (such as sales receipt showing the serial number), Apple may be prepared to assist - otherwise they and nobody here will be able to help you.

If you have the necessary documentation, Apple have introduced a portal though which you may be able to request release of an Activation Lock:

How to remove Activation Lock - Apple Support & start an Activation Lock support request.

However, recovery of your own AppleID credentials should be possible…

If you have forgotten your AppleID or associated password, these can be recovered here:

https://iforgot.apple.com/

To recover your credentials, you’ll need access to any one of the following:

• Your primary email address mailbox that corresponds with your AppleID
• Any of the secondary/recovery email addresses that should be configured for your AppleID account
• Any of the trusted telephone numbers (fixed line or Cellular/Mobile) that are associated with your AppleID

More information about recovery of your AppleID and associated Password:

If you forgot your Apple ID - Apple Support

If you forgot your Apple ID password - Apple Support

Activation Lock is intended as a theft deterrent, and is intentionally difficult to bypass.

We can’t help, and absent sufficiently convincing proof, Apple won’t.

Regain access to the Apple ID holding the lock, return to the seller for remediation or refund, or provide proof to Apple.