Verify the OpenPGP signatures provided by the distribution, keep up to date with security issues and join the distribution to help out with hardening it.

Some links for Debian:

https://www.debian.org/CD/faq/#verify https://www.debian.org/CD/verify https://wiki.debian.org/Hardening/Goals https://wiki.debian.org/Hardening/RepoAndImages https://security-tracker.debian.org/tracker/data/report https://www.debian.org/doc/manuals/developers-reference/pkgs... http://security-team.debian.org/