Head files in C contain definition of methods. One important technique to learn is applying Header Guards to your code. Those look like this :

#ifndef YOURFILE_H
#define YOURFILE_H
// your declarations here
and ensures your header file only gets included once. The first time it's included YOURFILE_H will be defined, so the next time an attempt is made to include your file, the preprocessor will just skip everything and go straight to #endif