If you have never done any web design or development, then I recommend you start from the beginning.

If you can dedicate the time, several hours per day, then you can get a general understanding within 6 months.

You will get more proficient as time goes by and if you practice and develop consistently.

Here is what you should learn in this specific order.

HTML – is the skeleton of a website. Every website uses HTML which is why you should start here. It’s not that hard.
CSS – is what makes a website look nice. I consider it the skin of your website.
JavaScript – is both a client-side & server side language that adds some dynamic features to your website. Kind of like muscles.
MySQL – is the database that is used by the vast majority of websites found online. This is like your brain.
PHP – is a server-side programming language and is the most widely used language for websites. This is like your Brain, heart & muscles.
WordPress – is the most popular CMS Content Management System in use today. It is built with all the above languages and Database.
Resources to use:

W3Schools Online Web Tutorials
Mozilla Developer Network
PHP Manual – Manual
MySQL Documentation
PHP: The Right Way
Hacking with PHP
Getting Started | Theme Developer Handbook | WordPress Developer Resources
Plugin Developer Handbook | WordPress Developer Resources
Hopefully, this helps.