For the next time:

Code:
apt-get update
apt-get install software-properties-common
add-apt-repository universe
add-apt-repository ppa:certbot/certbot
apt-get update
apt-get install certbot
Code:
certbot certonly --renew-by-default --webroot --webroot-path=/var/www/forum -d killtube.org -d www.killtube.org --email=lama12345@gmail.com
cd /etc/letsencrypt/live/killtube.org/
cat privkey.pem cert.pem > privkey_plus_cert.pem
/etc/init.d/lighttpd restart
https://certbot.eff.org/lets-encrypt/ubuntuxenial-other