Q:

Including external Header files in programs

Being using Windows for most of the time and deciding to start programing in ubuntu,I would like to know how I can include the external header file in my programs.Do I download them manually and paste them into the folder wheres the file?Please be kind...

A:

If you have a program print.c that prints a text:
/* print.c */
#include <stdio.h>
#include "print.h"

int main(void) {
    puts(TEXT);
    return 0;
}

and a header file defining that text:
/* print.h */
#define TEXT "Hello World!"

Then you usually put these files in the same folder. For external dependencies, it depends. Many external dependencies should not be downloaded from a website, but installed from the Ubuntu repositories.
For instance, the OpenGL headers are available from libgl1-mesa-dev (which installs mesa-common-dev). 
This makes the headers available in /usr/include/GL/gl.h.
In other cases where the header file is not located in the same directory, specify the different include directories while compiling. Example where you have a directory external-deps containing header files:
cc -I external-deps source.c -o output-program

If there are libraries involved (example for an OpenGL program using libGL):
cc -I external-deps source.c -o output-program -L path/to/libraries -lGL

