There are many fancy algorithms worth studying. This is just my naive, sub-optimal, bruteforce Friday evening implementation.

#include <stdio.h>
#include <string.h>

int strStr(char* haystack, char* needle) 
{
  // return value
  int pos = 0;
  // save the beginning of the needle
  char *nee = needle;
  // haystack offset
  int goBack = 0;

  // save these for performance
  int haylen = strlen(haystack);
  int neelen = strlen(needle);
 
  // edge cases
  if (haylen == 0 && neelen == 0) return 0;
  if (neelen == 0) return 0;

  // bruteforce
  while (*haystack) {
    while (*haystack++ == *needle++) {
      goBack++;
      if (*needle == '\0') return pos;
    }
    haystack -= goBack;
    goBack = 0;
    needle = nee;
    pos++;
    // performance boost
    if ( (haylen - pos) < neelen ) return -1;
  }
  return -1;
}