If something is "In the way", means that it is hindering or blocking movement. "We tried to get into the garage, but Ted's motorcycle was in the way." "On the way" means that something or someone is traveling or moving to a destination. "Tom was late, but called us to tell us he was on the way." "On the way" can also mean that something is encountered during a trip. "We went to our parents and stopped to eat on the way."

Hi ,I have some doubts about the preposition,but in this web page explans this.So ,you look it: http://owl.english.purdue.edu/owl/resource/594/02/