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?

>>23
there is my ver function from >>24 from /prog/

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