Let's Encrypt is offering free and trustworthy SSL-Certificates. Certbot will support you by adding those to your apache hosts.
To install certbot we need to add "backports" to /etc/apt/sources.list by adding the following line:
deb http://ftp.debian.org/debian jessie-backports main
#Apache $ sudo apt-get install python-certbot-apache -t jessie-backports
$ certbot --apache
Just follow the given instructions. Certbot will automatically change the vhost-configurations.
For automatic renewal we will have to define the following cron-task:
* 1 * * 1 certbot renew --quiet