Answer 1

I'm not good at it, but maybe try this
https://stackoverflow.com/questions/772041/using-c-library-in-c-sharp

Answer 2

You need to had the compiled library to your project by adding its reference (usually it's a DLL file) Then at the beginning of the file where you want to use that library, you will write:

using [library name] ;

Where [library name] is the actual name of the library you want to use whitout brackets.