After purchasing & Installing the SSL certificate, you will need to force SSL usage on Drupal / WordPress.
Inside the .htaccess file, you will need to add something like this (replacing your domain name as needed):
RewriteCond %{HTTPS} off [OR]
RewriteCond %{HTTP_HOST} ^domainname\.com*
RewriteRule ^(.*)$ https://www.domainname.com/$1 [L,R=301]
These rewrite conditions should be placed after the following lines:
<IfModule mod_rewrite.c >
RewriteEngine on