Consider that you have installed SSL Certificate on your site and now your site is visible using https:// protocol. If you to redirect all http:// request to https:// then you can do this by using .htaccess file.
Write following content in .htaccess file and place this file in your DocumentRoot Folder of your web site.
RewriteEngine On RewriteCond %{HTTPS} off RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]