How do I unlock iPhone's Activation Lock?

You will learn from the link kb1951 provided, you need to know the Apple Account and Password to remove Activation Lock. The question I have is do you know the Account and Password or did you buy a used iPhone from someone, which came Activation Locked? If you bought a used phone, you can't remove Activation Lock, unless you know who originally locked the phone. If you don't know them or can't reach them, the phone is useless to you. Apple won't remove the Activation Lock on an iPhone unless you can present them the original purchase receipt from Apple or an actual Apple Authorized Seller, which shows the device serial number. If you don't have that, you have a useless phone you can never use.

So, if you did buy a used iPhone from someone read this --> The All Too Common SAD Reality of Buying … - Apple Community

Start here: How to remove Activation Lock - Apple Support