Go back to just C++ and learn more. In 4 months you have barely touched the basics. If you don't know the difference between a public and private member function, then there's more to learn first, before diving into a library that requires you to have a basic understanding of C++.
I can't stop you from playing around with SFML, but keep in mind that the SFML forum isn't here to teach you C++, it's not even here to teach you SFML, but it's here to help you and support you using SFML. Plus the huge catalog of threads may give you solutions to problems you may run into.
For general C++ questions use StackOverflow or similar.
As for your problem, get your C++ book and read or hopefully re-read the section about access specifiers.