int x;
int length = strlen(orignal_string);
//length will be from 0 to 20 depending on the string actually in original_string. DON'T use 21, which is the maximal possible number of char in original_string, but may not be the actual number of char in original_string.
for (x = 0; x < length; x++)
{
reversed_string[x] = original_string[length - x];
}
//now reversed_string is a non-null terminated char array with lenght number of char in it. You need to add a null char at the end of the real char in reversed_string to change reversed_string into a string. The value of x == length at this point and represents where the null char should go.
reversed_string[x] = '\0';