Hiya.

Depends what you want to do with it. You won't ever figure it all out, it's incredibly deep. I'd say do as much as you feel comfortable with, and then experiment a little. Try running a mail server on it. Do some programming. Run a database server.

If you want to get into the command line stuff, learn how apropos and man work, read some old stuff - I suggest starting with Linux Gazette, it goes way back. Remember of course that things have changed a lot, but you will surprise yourself with all the things you could do since the mid 90s. It gives you an idea of where it's coming from.

Above all, get very familiar with Google! And have fun.