[Oni]

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.

like SQLi and XSS?

If you're wanting to learn about SQL injection, you should have knowledge of MySQL. If you're wanting to find XSS, you should have knowledge of how websites are made. PHP would be another language I'd suggest, as they all go hand in hand.