I would recommend setting up a local install of WordPress. From there you can play around with different themes and plugins. I would take a look at the code behind the default themes to see how they are built.
The best way to learn is by doing, so I would think of a site you can make and build it from scratch. Maybe a portfolio site for you or whatever. I don’t think you really need to go through courses, just google what you need to know, there are plenty of free resources out there! Between this site and google it is pretty rare I don’t find an answer immediately.
Here are a few resources to help.