If you bought it used, you have to go back to the original owner (who may not be the person you bought it from) and ask him to remove his Google account.

If you bought it new, bring it to your carrier (or wherever you bought it) and tell them that it's not a new phone. (Manufacturers don't put Google accounts on phones and they don't enable the lock screen. Regardless of what some saledroid may try to tell you.)