For linux newbies and experts, a really good linux resource is linuxquestions.org. It's a massive forum with a lot of people posting every day. Very easy to get help there.
Another good linux-for-newbies page I just found the other day is
http://www.linux.org/lessons ... it looks really good and in-depth.
To reply to Jacerra, Linux is hacked as much as windows. There aren't any widespread worms like there are for Windows, but it has its share of Sendmail, Bind, Apache ... exploits that I see tripped everyday on the networks I monitor.
TCP/IP is definitely something to know ... how else are you supposed to know how to craft those IDS-evading TCP packets? Or to decode those logged binary TCP packets

Another good/interesting "language" to learn, is shellcoding. There's a pretty good article about it on neworder (neworder.box.sk).