Q: i forgot my login password at start screen

Lion / Mountain Lion

User Password?

http://support.apple.com/kb/PH11390

Administrator Password?

Follow the instructions in the beige and green boxes.

http://pondini.org/OSX/Password.html

Note

You should now be able to log in with the new password, but you won't be able to unlock the Keychain. If you've forgotten the Keychain password (which is ordinarily the same as your login password), there's no way to recover it. You’ll need to reset your keychain in the preferences of the Keychain Access application.

http://support.apple.com/kb/TS1544