ZTE z812 is also known as ZTE Maven and it's an Android mobile phone. To bypass the lock, a code needs to be entered. If you don't remember the code, then you can enter the email attached to phone to recover it. It's a 50-50 chance to succeed, because in most of the cases, that recovery form doesn't appear.
Another way is to factory reset the phone, but this procedure might delete your content (music, videos, pictures, settings, files, etc).
To enter recovery mode, turn off the phone then press Volume Up + Power button until the Android logo appears, then confirm with Power button. Select Wipe Data/Factory Reset and confirm with Power button. It might take a while for phone to start, because the OS is being reinstalled.