We will secure the WordPress site with the free SSL – Letsencrpyt.

Note : Applicable only for domains having Apache as Web-server

Make sure you have git installed 
# sudo apt-get install git
# cd /tmp
# git clone https://github.com/certbot/certbot
# cd certbot

Now request the cert for your domain example.com

./certbot-auto certonly --webroot -w /opt/bitnami/apps/wordpress/htdocs/ -d example.com

Enter the requested info from certbot.

The requested certs would be generted in the /etc/letsencrypt/live/domain.com directory.

Link the certs for the correct rendering by Apache.

# sudo ln -s /etc/letsencrypt/live/domain.com/fullchain.pem /opt/bitnami/apache2/conf/server.crt
# sudo ln -s /etc/letsencrypt/live/domain.com/privkey.pem /opt/bitnami/apache2/conf/server.key
# sudo chown root:root /opt/bitnami/apache2/conf/server*
# sudo chmod 600 /opt/bitnami/apache2/conf/server*
# sudo /opt/bitnami/ctlscript.sh restart apache

To force users on https :

Open /opt/bitnami/apps/wordpress/conf/httpd-prefix.conf and add this on top :

RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule ^/(.*) https://%{SERVER_NAME}/$1 [R,L]
# sudo /opt/bitnami/ctlscript.sh restart apache