Little example:

main.c:
------------------------
#include"sum.h"
int main()
{
int a = sum(5,5);
}
------------------------

sum.h:
-------------------------
int sum(int,int);
-------------------------

sum.c
-------------------------
int sum(int a,int b)
{
return a+b;
}
-------------------------

g++ main.c sum.c -o sum

[quote name="Thomas Fritz"]Little example:
Hi thomas thanks for your answer but regarding the { sum.h} , where iam supposed to write it, i wrote the main in a file and the function in a separate file but dont know about the sum.h

Just put the sum.c and the sum.h in the same directory than the main.c for now.
Later, if you want to have them in different directories, you'll have to write a makefile or to include sum.h with the right path.

Sure you can,
you can make 2 .C files, one include .H file and something like:
#include "secondfile.c" in the first one.
then you can compile them