determining whether someone is a new visitor or not is not that strait forward.
A cookie could be used, but keep in mind that once the cookie expires, or the user clears the cookies from their browser, then they would be considered a new user. Also, it the user used a different browser, they would be considered a new user.
You would need to add a check to the login page to see if the cookie is set, if not, send them to the landing page, on the landing page, set the cookie. That way, once they see the landing page, the cookie is set and the login page will allow access.