This article will guide you on how to setup your server.
SteamCMD is required for this, you can find information on SteamCMD here: https://developer.valvesoftware.com/wiki/SteamCMD
You can use SteamCMD to download the Avorion server files like this(this assumes you have followed the link above):
Start SteamCMD
# cd ~ && ./steamcmd
Login anonymously
Steam> login anonymous
Set the folder where you want the files to go
Steam> force_install_dir /enter/path/here/
Download the server files
Steam> app_update 565060 validate
Exit SteamCMD
Steam> quit
Change your directory to the path you chose
# cd /enter/path/here
Run the server
# ./server.sh
You can find the server files(server.ini, logs, admin.xml, etc) here ~/.avorion/galaxies/avorion_galaxy
If you're using ssh you can always use screen to keep the server running in the background, you can find more on screen here: https://wiki.archlinux.org/index.php/GNU_Screen