hmm Critter gave some good questions... You should take a look at the OSI model just for starters...Learn how to protect a Linux box with iptables..Learn how to navigate through a nix box..Learn how some exploits work.. Build your own LAN..Learn different network topologies.. Run Nessus on your systems...Learn how to install software on a linux box..Learn how to compile the software from source... Just my 2 cents