So, if I understand what you mean, you have a string in the form "H##" where # is a digit. You want to interpret these digits as a single integer and discard the leading 'H'.
To quote @anon73444976,
Try something like:
char input_buffer[] = "H15";
char output_buffer[20]; // big safety margin
sprintf (output_buffer, "Num: %d\n", atoi (input_buffer + 1));
Serial.print(output_buffer);
If you want DOS-style line endings (emulating the behaviour of Serial.println()), add a (sigh!) '\r' before the newline.
Bye bye String class...