[Dyme]

I'd recommend learning about MySQL and then HTML/Javascript.

How is HTML useful... also why is MySQL his first priority. I would say PHP is much more important above anything.

Also something that hasn't been mentioned - become familiar with linux systems. This will be very handy once you start learning post-exploitation.

Once you have a basic understanding of the system and code that you are trying to hack, then you can learn the process of how to actually hack it. A good place to start doing that is https://www.owasp.org/ (for Web Exploitation).