@Niko, you may want to start by reviewing the information in this thread.
A quick suggestion will be to start with a book (e.g., by Pohl) or a university guide from their computing department (many available online) that are unfortunately dated to late 1990s for C++ for Fortran programmers. Then move on immediately to modern C++ (C++20 and C++23) books and materials in conjunction with modern Fortran and kinda develop a guide of your own interest. The work on LFortran can be of help to you.
Ultimately the call is yours keeping in mind your colleagues and your computing domain(s), etc. as to whether you want to proceed with either all C++ codebase or a mixed-language C++ and Fortran codebase.
Both are workable but the latter requires care , especially with C++ exception handling and with ensuring proper runtime and libraries that use C as the bridge layer. So many teams out there prefer the former. There are few circumstances now where it can be all Fortran.