There are a number of different potential topics you could read up about:
- Wireless hacking seems to be pretty popular here, though its not my cup of tea. If that interests you start with WEP cracking then move to WPA
- Web application hacking is interesting. Theres a page of Deliberately insecure web applications listed on the IronGeek site. Check out these apps, and Google for tutorials on each.
- Network pen testing is good fun. Have a look at the De-ice cds and Damn Vulnerable Linux.
- Buffer overflow exploitation. If you have a look at AnActivists Pentesting Documentation thread my last post there has some links. That thread is also a good read generally for newbies. Also check the bindshell shellcode thread in the Pentesting forum, there's a link to a bunch more references on the wiki.
- Exploits and exploit frameworks. Check out the exploits at Milw0rm and Metasploit. Id suggest learning about buffer overflow exploits first however so you understand what your doing.
Google any of that and you should be able to find good guides or tutorials on the subject.