Depends on what you want to do with *nix,learn how to use it, administrering it, shell programming.
There are so many things you can do.
Best thing to do is install a *nix OS on your pc and depending on what you want to do, google it and try to find your way out.
And if there is a command you want to learn how to use try the man pages. # man <command>
``The wise man doesn't give the right answers,
he poses the right questions.''
TIMTOWTDI