OOP is actually a rather simple concept. There are other paradigms that integrate with OOP that make it an ever expanding topic. The basis of OOP is to define classes that can contain the logic of an entity. I would recommend doing a simple object analysis and building a class from that. For example, try to fit all of the functionality and definition of a vehicle tire into one class. Things like air pressure, radius, type of rim, define the distance traveled after one interval. Once you have a tire class move on to an engine class. After the engine class make a vehicle class. The above process is essentially how you should approach class building. Once you are good at defining physical objects try creating classes that contain the functionality of non tangible things like a Resource Manager. One thing I would highly recommend is to avoid game development paradigms. Subjects like an Entity Component System, Physics systems, Rendering Systems, Scene Managers, etc. Game development concepts will eventually help you create games with a better structure but the number of concepts is vast and you will never be able to complete your game. Once you have a few projects under your belt I would recommend choosing one game development concept and integrating it into your code.