I learn by doing. It’s a struggle at first because I know so little, but it gets better. I start by picking small, attainable projects that I can figure out. As I gain skills, I can handle more complex projects. This can take a very long time if projects do not present themselves. You can always create projects, but real ones are better.

Believe it or not, I learned a lot by helping others here. When a question comes up where I don’t know the answer, but think I can figure it out, I give it a try. Sometimes I fail, but I still learn something. When I succeed, I’m able to help someone else out who is usually grateful and I learn something in the process.

I suggest you familiarize yourself first with front-end web development in HTML5 and CSS3 (possibly get educated in LESS and SASS for CSS, as well. You’re also going to want to learn javaScript, jQuery, and other forms of javascript. There are always new breeds of javaScript frameworks being invented, so it’s hard to keep up. When you’re well versed in building front-end for themes, then you’ll be working with php to insert the calls and functions that underlie the them, pushing/pulling data and display functional options in the theme.

Here is a good article from Elegant Themes (great blog from a qualified WP development shop) that will get you started thinking about how you want to attack your learning.
https://www.elegantthemes.com/blog/resources/11-online-places-to-learn-wordpress-inside-and-out-paid-and-free-options

Remember that with development comes “design.” You can know all the development processes, but then you need a designer to design the layout for you to slice in Photoshop and start building in HTML and CSS.