This will sound funny at first, and some people will think I'm being facetious, but I'm not.
- Find another experienced admin, sit down, define a goal of some sort, and attempt to reach that goal while they provide guidence.
Unix (and BSD and Linux and who-knows-what-else) was originally an environment that encouraged the programmers and administrators to communicate with each other. Linux is a fairly decent approximation of that (not 100% but very close).
In my not so humble opinion, the Unix/Linux/BSD admin is best taught through a form of oral tradition, where knowledge is verbally passed to the learner by those who are much more experienced. In every case that I have encountered, this method of learning Unix (and its cousins) has been the most informative while providing the most amount of insight to the learner, especially insights that you cannot obtain from simply reading a book and forging forward. The reason for this "inefficient" method of learning is, at some point, someone somewhere made a decision about something, but it's not formally documented (read the header to that link). This means it's easy to sit there and scratch your head about "why is X done the way it is" when in reality, there was probably a very good reason for that. Having someone who's been there, and knows the history of how-it-came-to-be, helps to "fill in the gaps".
Yeah, you can sit down with a book, or (like I did), simply install it and bang your head a few hundred times while you figure it out the hard way. But there's a lot to be said for learning from someone else.