Gst for tour and travel agent

i am a tour and travel agent. mainly i booked air/train ticket..example  if i booked  train ticket then my gst number will be provided in IRCTC website or if i booked air ticket in makemytrip then gst number will be provided in makemytrip website... if i provide my gst number in IRCTC website for train ticket booking then IRCTC provide me ITC, in this case if i claim INPUT then in  my invoice OUTPUT will be mandatory or only 18% GST for service charge will OUTPUT.