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?

How can I fix this?
Actually return a value from main as specified by the declaration.
Don't use gets().
Don't use C.
Dont use puts().
Don't take any more computer science classes.
Don't continue breathing.