How to add an extension

Hi This is my problem: I read that I should run ext_skel script with name of my new extension. I did it. But I dindn't find any *.c file. I don't know how to write it. I know that i should modify another files too (internal functions.c) and create *.h file. How can I do it ? I can add my own functions by modyfing already exist modules. But I'm interested in writing my own modules. Thank for any help Adam