Why do I need to know an object-oriented programming language?
Because most of modern applications are written using an object-oriented programming language. Using an object-oriented programming language as a tool will save you a lot of time and cost when you create a modern application.
Alright! What should I do now?
Please read this "Jim Keogh and Mario Giannini (2004). OOP Demystified" book.
Is that all?
Unfortunately, the book is not sufficient although it already gives you the fundamental concepts of object-oriented programming. The book does not teach you how to create real world applications using an object-oriented programming language. We recommend that you read it first because it introduces basic concepts of object-oriented programming very well. It helps you to distinguish object-oriented concepts from procedural programing concepts very clearly so that you could read other books to dig into object-oriented programming in a right way. Please read this "RB Whitaker (2015). The C# Player's Guide" book to understand how to use object-oriented concepts to create real world software using a specific object-oriented programming language. After that please get - this "Joseph Albahari and Ben Albahari (2018). C# 7.0 in a Nutshell: The Definitive Reference" book or - this "Herbert Schildt (2010). C# 4.0 The Complete Reference" book and take a quick look at them so that you can refer to specific topics when creating real world applications.
I hear that there are many object-oriented programming languages such as C++, C#, Java, PHP, Objective-C, Swift, Python, Ruby, etc. How many object-oriented programming languages should I know?
As many as possible. However, in this stage, I recommend that you learn only C# or Java or C++. - If you are required or prefer to learn C++, please read this "Bjarne Stroustrup (2013). The C++ Programming Language" book. - If you are required or prefer to learn Java, please read this "Herbert Schildt (2014). Java: The Complete Reference" book. Then if you are interested in in-depth object-oriented concepts then please read - this "Matt Weisfeld (2019). The Object-Oriented Thought Process" book and - this "Bertrand Meyer (1997). Object-Oriented Software Construction" book and - this "Grady Booch et al. (2007). Object-Oriented Analysis and Design with Applications" book. They will help you understand the object-oriented concepts more deeply. After finishing the books please click Topic 5 - Introduction to Windows Programming to continue.