* You need to 'apt-get install' tvheadend first. As per the download instructions for ubuntu.
http://tvheadend.org/projects/tvheadend/wiki/AptRepository
* You need to enable the tvheadend system service in Systemd.
Which I'm sorry I can't help you with as I use Docker and not systemd. Here's some resources:
http://www.howtogeek.com/216454/how-to-manage-systemd-services-on-a-linux-system/?PageSpeed=noscript
https://github.com/tvheadend/tvheadend-packaging/blob/master/systemd/tvheadend.service
You will probably find that the tvheadend apt pkg installes that file ^^ `tvheadend.service` in /etc/systemd somewhere. But that i just a guess on my part.
Honestly I much prefer Docker myself. But that would be a steeper learning curve for someone like you who wouldn't really know anything about it yet.