Getting into drupal 8 from drupal 7 is hard. Especially if you are new to OOP, some articles about what is what will help you on that way. I suggest the following in that order
this should cover main terms about OOP in drupal 8 and services, containers, dependency injection which are real magic behind this new approach.
After that you need to learn about plugins, routing, controllers, event subscribers, how entity api works, how form api works and see how drush and drupal console can help you with all of that. With that knowledge you should be ready to go :)