Wily Werewolf (15.10) and newer:
You don't have to do anything. You are using systemd by default.
Trusty (14.04) and older:
You are out of luck. Ubuntu repositories do not provide the systemd binaries for versions before Utopic, despite the fact that their sources do generate the files. You can learn more about this in this answer.
add these ppas
systemd requires the directory
then install systemd
for more information go to this link