Let's Encrypt is a free https certification signing organization. Followed these steps here to install letsencrypt certbot.
The command I used to create a certificate for this site and the other sub-domains was:
This created the certificate.
Now test the renewal process by typing:
It worked perfectly. Now setting up the cron-job to do automatic renewals.
Add the following line in crontab:
here. Added the following lines to the nginx configuration file for this site:
After setting the cronjob the certificate renew happens but the server should be restarted in order for it to use the new certificates so also add the restart server command in the crontab.