First, You'll want to make sure the directory you're adding the .htaccess file to is set to AllowOverride All in your httpd.conf file.

Next, create the .htaccess file with the following contents

<IfModule mod_rewrite.c>  
    RewriteEngine On
    <IfModule mod_ssl.c>
      RewriteCond %{HTTPS} !=on
      RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
    </IfModule>
</IfModule>  

Now, assuming the .htaccess file is in your root directory, when a user visits http://localhost they will be redirected to https://localhost