Include/Import Header Files/Source Files (C++)

I’m a bit confused about when I’m supposed to import a header file or a source file, and where all the various links go…

Basically, I’ve got my one C++ file that compiles fine and all, and it includes one header file from a library I’m using (for Arduino src), but I want to include my own separate files. It works fine if I just do something like,

#include "file.cpp"

but apparently I’m never supposed to include a source file, and instead a header file. So, if I change that to “file.h”, it is all fine and happy including the header file, but that never includes the source file. Should file.h have an include to file.cpp? Should there be something in my Makefile?