Good article at Make: Take These Steps to Secure Your Raspberry Pi Against Attackers. Pretty much standard stuff but good stuff well worth considering even on a project SBC [single board computer].
Why is the SBC a target? Make suggests it might be used for distributed computing to aid Bitcoin miners, as a host for I’net attacks, or as an entry into your LAN.
Besides disabling the default user and access and creating your own, there are instructions about adding security to SSH access, setting up iptables for a firewall, and other system management tasks.