Best part of learning is to refer the official documentation.
1)Wordpress has documentaation for plugin developement refer https://codex.wordpress.org/Writing_a_Plugin
2) Also when I was developing a plugin I followed some other tutorials to other than the official documentation. I think below links would help you.. hongkiat.com/blog/beginners-guide-to-wordpress-pl… code.tutsplus.com/tutorials/create-a-custom-wordp…
Try to understand the hooks and filter concepts of wordpress properly it helps you a lot.