If you have forgotten the pw for google/gmail to log in, use this link to unlock google/gmail and reset pw https://www.google.com/a/yourdomain.com/UnlockCaptcha. After google/gmail pw reset try that pw in your phone. if that dosent work try pw as null. if that dosent work you need to use hard reset option using phone keys.
Check out a fun infographic that teaches you how to increase storage, enhance security, improve battery life and clean your device.