I have forgot my google account username and paasword so what i do to

You don't need to unlock your phone because you forgot the user and password. Especially since it's still on your phone, unless it's a new one. In which case you would have to create a new gmail account for that phone, since you can't remember the last one.

Unlocking is a process that can result in screwing up the phone if you don't know how to do it properly. But you can just google "Unlock Android phones" and find lots of results.