HTTPS is an important option for modern websites. With free SSL certificates from Let's Encrypt there is no excuses do not use it.
First of all, you have to need download Certbot install script:
$ wget https://dl.eff.org/certbot-auto
Give the permission for the file execution
$ chmod a+x ./certbot-auto
Then run it:
$ ./certbot-auto certonly --manual
The script will install all necessary packages and run certbot. Now you need to answer for a list of questions.