You have two options for turning off your server. You can send a command instructing the server to power down or you can perform a hard shutdown.
A hard shutdown is much like yanking your computer's power cord out of the socket while it's still turned on.
(If you are renting a VPS/dedicated/cloud server from a hosting provider, they should have provided a link to a web-based control panel that you can use to do a hard shutdown.)
Don't make a habit of turning off your server by doing a hard power shutdown.
You can potentially cause irreversible damage to your computer/server and its data if your improperly turn off your server while its writing data to your disk drives. A hard power shutdown should only be a last resort when your server is non-responsive.
WARNING: Abruptly rebooting your server may cause file damage and/or prevent programs from operating properly. This is much like pulling out the plug on your PC while its running. This should be a last resort if a graceful reboot is unsuccessful.
sudo shutdown -h now
sudo shutdown -h 0
sudo shutdown -h [time] "[Message]"
sudo shutdown -h 15:45 "Server will be offline for maintenance"