A graphical Seafile client is available for Debian, Ubuntu and derivatives, CentOs and RHEL. For installation, please follow the instructions in Install Seafile Client on Linux.
Users of other Linux distributions can install a terminal client to synchronize their libraries.
On Linux systems, Seafile is not automatically started at boot time. To start Seafile automatically anyway, open System Preferences and search for "Start". You should be offered to configure the startup programs. Add Seafile to it.
Due to the large number of graphical user interfaces, we are unfortunately unable to provide more detailed instructions. If you cannot find the configuration of the startup programs on your computer, search the Internet for the name of the interface you are using (e.g. Gnome3, KDE, Mate, XFCE) and "auto start".