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
now you have .key file and .csr files
Now upload your csr file in the below site
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”.
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 .