Mix in the same public_html folder of website your html, css files and files of wordpress is not very logical and can give you more of a headache.
000webhost allows more than one website for each user.
I advise you to do a new website (with your email and password, you can host multiple websites).
In this new website you can install wordpress, configure and test. And when everything is ready, you can migrate this new website to your current website.
Indeed, searching the web I found a possible solution to the question that started this thread, but is ONLY VALID if your website only going to stay the Wordpress CMS
PS: I will move this thread to "Website Builder" -> "Content Managament System
" because it is a more appropriate place.