How can I unlock an iPad if I don't have the Apple ID?

You can't! You must have the Apple ID and Password of the original owner who set the Activation Lock. Check these links...

Turn off Activation Lock - Apple Support

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

I’m sorry but you don’t. That is the activation lock and since it’s meant to be a theft deterrent system there is no work around.

Turn off Activation Lock - Apple Support