[Someone]

How do you learn? I would buy Stroustrup's "The C++ Programming Language" http://www.stroustrup.com/4th.html and read it back to front, but that's not for everybody (stroustrup.com is worth looking at, regardless of how you learn)

What do you know? If you know C, for example, it is different than when all you know is prolog.

Also: what version? C++ has seen significant updates that changed C++ best practices quite a bit. I would aim for at least C++14, but if you want to work on older code or on software for a system that doesn't have a recent compiler, that isn't a good idea.

Finally: Learning C++ isn't as hard as 'the Internet' say it is; historically, it is more that you constantly have to be on your toes when writing code, especially for code that constructs, copies or takes down objects in various ways. Modern versions are better in that respect; you still can eek out every bit of performance, but if you don't try (which you shouldn't, in the first version of your program) and use modern idioms, there are far fewer pitfalls.