Setting a good root password is the most basic requirement for having a secure system. See passwd(1) for some hints on how to create good passwords. You can also use an automatic password generation program to do this for you (see 「Generating user passwords」).

Plenty of information on choosing good passwords can be found on the Internet; two that provide a decent summary and rationale are Eric Wolfram's http://wolfram.org/writing/howto/password.html and Walter Belgers' https://web.archive.org/web/20030218000949/http://www.belgers.com/write/pwseceng.txt