Creation of C++ library dll. interface description?

Question

C++ / C#

You need to create a library to convert files to a specific format.
The goal is to create a library that will receive data, write it to 3 separate different files. At this stage, I learned how to create files, write down the data I need, and described the structure of the files.
Prompt, please, the good literature on the given question, available to the beginner.