Hi! I learnt how to work with classes, what is inheritance and what is polymorphism. Now I don't know with what to continue. Can you recommend me a good source to learn more about OOP? I have been searching in the Internet all night, but I couldn't find good one. Do you know any?
Definitely look into the books listed. Read the tutorials on this site, on tutorialspoint.com, cprogramming.com, and learncpp.com. If you don't learn from them then there is no site I can think of that explains it better.
I'd avoid books that old as they are outdated and some do teach methods that are no longer efficient. For example, when I got into programming back in '95 or so it was perfectly fine to do void main(){} and that is no longer a valid main function.
I'd be selective or find newer editions if they exist. Otherwise you will have to unlearn the bad habits you pick up from those books.
GoF is indeed old (it predates the first C++ standard!), but today's discussions of OO design assume you know what was written in that book (and, if they are about C++, they typically assume that you know how those patterns were applied to C++ in Alexandrescu's Modern C++ Design)
This really is a hard topic to discuss because every book has people for and against it so it is hard to say definitely what books to read. Same with sites, some are for or against sites too so that makes it hard to say one way or another without wondering if it was recommended in bias or not.