Not sure what you excatly mean.
To send emails you need to have a proper MTA configured.
Having said that, sending email is not specific to localhost in general, so my guess is that by localhost you mean localhost on WAMPSERVER which isn't by default configured to send emails out.
One solution is to use fake sendmail (http://blog.techwheels.net/send-email-from-localhost-wamp-server-using-sendmail/), which will then act as if Sendmail is installed.
As for "How can i change the option to only registered users can post in my site?"
You set this in thea Admin > Permissions.
It can be granulary set to Question / Answer / Comment.