Well how well do you know PHP and the other web languages. If you know a lot you can learn basic exploitation such as: SQLi XSS CSRF Cookie poisoning etc.. I'm no expert in web exploitation but I do know the basic gist of stuff. After you learn the basics, you should learn HOW they work and how to find them in PHP code. If you're auditing something like myBB you're not gonna find a simple SQLi vuln in the core of the code. You may want to dive deeper into more advanced techniques. I also have a collection of books here: http://bit.ly/eiv5gV
I know theres a paper in that tarball about going from SQL injection to full operating system control.