I've downloaded Xcode and i've done :
which "gcc"
which "ggc-4.0"
and which "c99"

all were found in /usr/bin/

however with the script

#include <stdio.h>

main()
{
printf("hello, world\n");
}

and when I, in command line use the command while in my home dir:

gcc cprog.c -o cprog

i have no output
just another line prompt

any help appreciated and i'm sorry if this is a terribly simple question

That means your code compiled into an executable without an error.

Type ./cprog to invoke the resulting executable.

i tried :

`gcc ./cprog.c`

and

`gcc ./cprog.c -o ./cprog`

nothing worked, thanks though

No.

fp means: to see what your code does when you run the compiled code: ie., get "hello world" on the screen.

./cprog <-- this is what you type, no "gcc" anywhere, then press return.

To explain this better:
You write the program into a file called "cprog.c".
Then after you execute "gcc cprog.c -o cprog", use the command "ls" to list the current files in the directory ("man ls" for more info).
You should see a file called "cprog". This file cprog was created when you did the "gcc cprog.c -o cprog", and this command means that you use the program gcc (gnu c compiler) on the c source file "cprog.c". Then the "-o" means to output the binary file to "cprog". If you hadn't used "-o", gcc would have generated a file called "a.out". ("man gcc" for more info)
Lastly, you can execute the file "cprog" by typing "./cprog" into the terminal. This means to check the directory "./" (which means the current directory that you are in) for the file "cprog". If you didn't specify that "cprog" was in "./", then the operating system would have assumed "cprog" was in "/usr/bin" or another directory (if you want to know type "$PATH" into your terminal).

Does that explain things clearly?