Have been using C++ for the past 4 years, I hope that I am eligible to answer this.
I studied C++ as a part of my high school Computer Science and I used a book by Sumita Arora. If I have to start from scratch, I would do this to master C and C++. I would recommend you to start C++ right away as there isn’t much difference between the two.
1. Learn the basics of C++ by taking a course from Coursera / EdX/ Udacity/ NPTEL. I am not recommending any course here as I have not taken any C++ online course.
2. Buy a book. I would recommend this book by Bjarne Stroustrup. (Buy The C++ Programming Language, 1e: third Edition Book Online at Low Prices in India ). You need not study this book line by line but just keep it as a reference.
3. This is the most important step. DO A PROJECT! This is where you will actually “learn” C++. This is because once you do a course you just know how C++ actually looks like. It's like having a costly rifle and not having bullets to actually use the rifle.
This step is the one which will make you understand C++ better. You will not know many built-in functions and might need to google a lot. You will use StackOverflow, quora, other sites to learn a lot of stuff as you get stuck in the project. This will not only solve your problems but also gives you multiple ways to implement the same thing. Simple tic-tac-toe, hangman game will do.
Since I have programmed in the wrong way in my school I would recommend you to read the points below to not make the same mistakes I did.
Never use Turbo C++ / Dev C++. Use Codeblocks(Downloads) or Visual Studio to code. Codeblocks is the best for both C and C++ for beginners.
Never copy code from websites. This gives you an illusion that you know the concept but next time when you encounter the same problem you will look for a solution again which wastes time.
Never give up. You will initially take even a day to fix a bug, but be patient and keep moving.
More you code, the more proficient you will be. Also I wrote this answer just to tell that “Online” in your question does not mean only Online Courses. Getting help during a project also comes under “online”.
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