Hi friend
To use multiple files structure for c is very easy.
First of all, write multiple C files.
then, if you are use MPLAB
then go to project window and right click on source files.
select Add files, and add your all C file here.
Take care about Global variable.
If you define Global variable in one C file(like int Value = 100) and if you want to use that variable in other c file
you muse redefine that variable with extern keyword(like extern int Value).
Hope this is help you
Shyam