Start with a blinking led, and try other examples. You will find the answers along the way.
Arduino use C++, but also the AVR C-code can be mixed with it.
You can create new C++ files (use just a name) and also header files (name them as *.h) to your project.
Use the drop-down on the right of the tab bar in the Arduino IDE.
You can even copy *.c and other *.h files in the folder of your project.
To use a library, the library must be present, and the include file must be included.
You don't have to include your own include files.
But sometimes you do. If the compiler uses a wrong order, you can also include your own include file in the first sketch.