#include <stdio.h>?
 

almost all c code include from stdio.h, string.h..

but where does stdio.h stored.

if you search the directory there is alot all named the same, how do you know which one to use when make?

Your compiler knows which directory is your default include directory. For instance, gcc looks in /usr/include for header files so when you do #include <stdio.h> it's actually including /usr/include/stdio.h

Knowing which to use is a matter of reading the man page of the function you want to use, and maybe the doc's of an external library you may be using.

Knowing which function to use may be a lot more tricky to find out, especially if beyond the usual found in the standard headers like string.h stdlib.h, stdio.h....