Knowledge of OWASP top 10 is very helpful. PHP knowledge is also helpful on a large portion of the boxes.
Get familiar with Linux if you aren’t already, as you will need to be able to notice things that are not part of a default Linux installation. I would suggest grabbing a cheap VPS (from somewhere like DigitalOcean) and setting up Apache/MySQL/etc. A good project would be to set up your own webserver with a Wordpress installation. Once you have that set up, you will be able to compare HTB boxes to your own to see what is out of place.