if you want to use SSL certificate for the site and that to free ssl certificate provided by the vendor .they will give it for free for 3 months and if you want to re-use every time your have to renew the certificate .
you can generate CSR and Private key using below website . Just enter your site name and fill other fields .if you want to use www. click on the check box
https://www.freesslcertificate.me/csr-ssl-certificate-signing-request/
now you have .key file and .csr files
Now upload your csr file in the below site
https://www.freesslcertificate.me/ap/certificate-create/
on the next page you will get 2 files . you need to upload these files to your site . In my case it is Cpanel . I uploaded to /home/user/public_html/.well-known/acme-challenge”.
STEP 3
Click on the next in the above url it will complete the site verification .on the final step 3 you will get Certificate , full chain and CA bundle .
now go to your website and replace .key, Certificate and CA bundle then complete the certificate installation . Now verify your website with new certificate validity .