Use this method while you are learning the in's and out's.
Create one header file, this will contain ALL function prototypes and any enums or typedefs you might have. Includes this header in all your .c files.
so you header might look like this:
Then your .c files would have
int foo (void);
void bar (int *, char);
One thing I like to do is to add comments in the header that specify which file the function is in. EX:
/* ... */
Hopefully this helps.
int foo (void); /* In foo.c */
void bar (int *, char); /* In bar.c */