Thanks for your reply. I guess I tend to shy away from things like tuts4u and crackmes.de because it seems questionable legally, although I haven't looked into it much. I mostly "reverse" my own code in gdb just to understand whats going on in the assembly. These types of things are hard to show in a resume, so I'm not really sure how to make myself an attractive job candidate to some of these antivirus companies.
I don't see anything illegal in disassembling shareware or any other type of software. Compare it with disassembling your smartphone/alarm clock/hand watch/.. to find out how it works. Why should it be illegal?
"circumventing a copyright protection mechanism"? (or whatever the wording is in various laws.)
What you can do? I would start from tuts4you.com, teach x86 assembly language, download a degugger and/or disassembler and dig something or follow tutorials.