The Codex is the bible of WordPress, it’s well-documented, easily understandable and comprehensive. It covers all aspects of WordPress; How to Use WordPress, Themes Development, and Plugins Development. As our concern is plugins development, you should focus on this part. If you already have used WordPress before, I advice heading straight to the Writing a Plugin and Plugins API sections. After knowing the basics of plugins development, why not to go deeper and learn about more advanced topics. Luckily, we have the Plugin Handbook which covers almost all of the plugin’s APIs. I recommend reading it and to try out the code snippets on your WordPress development environment.
Plugins Source Code
There is no shame to stand on the shoulders of giants, especially that WordPress is an open source software. You can read the source code of popular plugins and learn new development techniques. There are great developers out there and they already made high-quality plugins. Reading other’s source code is a general tip where you can learn how programmers do certain things and how do they stick to the coding standards.
Write a Plugin
This is the most effective way, practise. That’s it. Write down some plugins’ ideas and start writing a plugin. If you didn’t know how to implement a feature, try to google it. If Google didn’t come with results (which is unlikely), ask questions on WordPress Development Stack Exchange. Other developers will try their best to help you out. The more you write code, the more experience you will get. You will become more familiar to solve common issues and problems while developing plugins. And if you think you made something useful, do not hesitate to upload it to wordpress.org, others will benefit from your plugin and your portfolio will be enhanced.
Whatever you think, if you are a good PHP programmer, then developing a WordPress plugin is not hard as you would think. It’s easy and straightforward. All what you have to do are to read the docs, read some code and start developing.