Without putting too much thought into this: Go for web application security next. That is, security problems with PHP(or other server-side scripts), SQL and HTTP. Alternatively go for software exploitation, which will lead you to C and some understanding of Assembly, machine code and shell code.
If you go for web application security I would recommend doing some PHP project(s) and read up on SQL injection, XSS, RFI, LFI, path disclosure, faulty checking of user input types and CSRF. We have a few of the topics in the tutorial section here. Some you might have to google.
If you go for the software exploitation I would recommend understanding.. Pretty much everything from when an executable is loaded by the OS to the executable exits. Some topics: Memory management, stack, heap, buffers and array sizes leading to buffer overflow attacks and alike. A good paper on buffer overflows is Smashing The Stack For Fun And Profit, Evilzone mirror:
http://evilzone.org/docs/smashstack.txt