Need help fast. I sucks ass at programming.

int main()
{
    char s[50], s2[50];
    int i,j;
    printf("type string");
    gets(s);
    for (i=0; s[i]!= '\0'; i++);
    for (j=0; j<i; j++){
        s2[j]=s[i-j];
    }
    puts(s2);

}
 It was suppose to "invert" the string. If I type "abcd" it was supposed to come out as "dcba". I fail at life.
How can I fix this?

there is my rev function from problem 55 from project euler


void rev(char *s, char *sr) {
  char *p,*pr = sr + strlen(s);
  *pr-- = 0;
  for(p=s;*p;p++,pr--)
    *pr = *p;
}