Factory reset

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

That won’t get you around Activation Lock, though.

Here is a general sequence, which also describes remotely-releasing Activation Lock:

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

To restore the phone, see the steps provided in this article:

• Restore your iPhone, iPad, or iPod to factory settings - Apple Support

If you forgot your Apple ID password, take a look at this one:

• If you forgot your Apple ID password - Apple Support