I am reading the material to build my email server.
https://www.digitalocean.com/community/tutorials/how-to-set-up-a-postfix-e-mail-server-with-dovecot

Here is a something confused me in the material:
Setting up SSL certificate
For SSL, you need a certificate and a private key. In this tutorial, we're going to assume that the certificate is saved in /etc/ssl/certs/mailcert.pem and the key is saved in /etc/ssl/private/mail.key. Make sure the key is only readable by the root user!

the certificate problem :
we're going to assume the certificate is saved in /etc/ssl/certs/mailcert.pem

In my server
ls /etc/ssl/certs/mailcert.pem

ls: cannot access /etc/ssl/certs/mailcert.pem: No such file or directory
My problem is :
How to create the mailcert.pem myself?