If you're not familiar with classes and header files you're going to have fun implementing anything in C++. If I were you I'd wrap my head around includes and abstract data types and then use libcrypto which is part of the OpenSSL
] distribution. It's a C library rather than a C++ one but it's got some very efficient implementations of most of the common cyphers in it.
It might also be a good idea to use a programming language you're comfortable with. C and C++ can be very unforgiving for self taught neophytes.