How to use function ?

#include<stdio.h>

#include<conio.h>
int header();
int sum;               /* This is a global variable */
int main( )
{
int index;
header();      /* This calls the function named header */
for (index = 1;index <= 7;index++)
square(index); /* This calls the square function */
ending();
getch();       /* This calls the ending function */
}


header()               /* This is the function named header */
{
sum = 0;       /* initialize the variable "sum" */
printf("This is the header for the square program\n\n");
}


square(number)         /* This is the square function */
int number;
{
int numsq;
numsq = number * number; /* This produces the square */
sum += numsq;
printf("The square of %d is %d\n",number,numsq);
}
ending()               /* This is the ending function */
{
printf("\nThe sum of the squares is %d\n",sum);


}