uhm... are you going to admin a server or do you just need to learn to use linux? as in.. using linux.
for one.. i'd make sure you understood how to navigate the command line and how the system actually worked before i even remotely began to think about putting my computer out there on the internet.. it's only as secure as it's admin.. and if you don't know anything, guaranteed you will be subject to an owned machine. think about that before you try it.
secondly... i recommend anything from
www.ora.com ... however the best way to learn is simply to use it...
www.linuxdoc.org