John started by learning about the basics of web application security. He discovered that web applications, despite their seemingly innocuous nature, were vulnerable to a wide range of attacks. He learned about the different types of attacks, including SQL injection, cross-site scripting (XSS), and cross-site request forgery (CSRF). The Web200 PDF provided him with a solid foundation in HTTP, HTML, and web application architecture, which he realized was essential for understanding how to identify and exploit vulnerabilities.