How can I unlock my iPhone if I forgot iCloud password

If it's a second hand iPhone then ask the previous owner to help.

FYI: iCloud is an alias to the owner's Apple ID so it has the same password.

Activation Lock is a theft deterrent feature with no bypass or workaround.

Without the owner's authorisation (Apple ID & password), no thief or anyone including you can re-use the device.

You can recycle the device responsibly